92264738-Ccnp-Route-Exam-Guide-v2-5.pdf

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642-902 Exam Guide No filler.

No hype.

Exam-focused.

“A portable, comprehensive guide with everything you need to get up to speed and pass the ROUTE Exam - the first time.”

www.ccnpguide.com

The Online CCNP ROUTE

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Introduction

I started www.ccnpguide.com as a way for me to capture technical notes as I prepared for the three major CCNP Exams – SWITCH, ROUTE,

& TSHOOT. As I began sharing my notes with the world, I immediately started to receive feedback on the SWITCH exam‘s focus areas and

how difficult it was. What I realized was that the exam prep resources available (read: Cisco Press Books) were not even covering all of the

exam topics, including some that you were required to configure in live simulation scenarios. First-time fail rates seemed normal and a big

part of that was because the some of the simulation scenarios required you to know some extremely specific protocol configuration details

that most network professionals just wouldn‘t know off the top of their heads.

I began to tailor my notes to include topics that were not being covered in ―official‖ exam guides and trimmed down those that just were

not necessary. The feedback was overwhelmingly positive from the online community! The problem is, of course, that the notes were not

formatted well for off-line consumption and didn‘t include enough lab/scenario-based examples.

This guide is an answer to the countless requests to create a portable, comprehensive, and exam-focused ROUTE prep guide. I‘ve refined

the online notes even more to focus exclusively on exactly what you Cisco expects you to know on exam day. I have also included a

Simulation Scenarios section at the end. Lastly, Exam Takeaway notes are scattered throughout the guide to help connect you with the

most important topics and study suggestions.

Here‘s my recommendation. Read through this manual a few times and make sure you understand each chapter. Pay close attention to the

Exam Takeaway notes and take them seriously. After you feel comfortable with the details in each chapter, go to the Simulation Scenarios

section and run through the three scenarios until you can solve them off the top of your head. That may mean running through them ten

times each, but trust me – you‘ll thank me when you sit for the test.

If you have questions, exam feedback, or want to reach out to me directly - shoot me an email at [email protected]. I promise

you‘ll get a response.

Best of luck.

Aaron

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Routing Basics 4

EIGRP 12

OSPF 38

Route Redistribution & Filtering 65

BGP 73

VPNs & IPSec 89

IPv6 94

Simulation Scenarios 108

ROUTE Shortcuts

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Routing Basics.

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Static Routes

In order for routers to forward packets to remote networks, they must know how to reach them. There are two options: static or dynamic

routes.

Static routes are manually configured on each router. They are used for a couple of reasons:

where there is only a single path to a network (a.k.a. stub network)

when connecting to an ISP and configuring it as a default (static) route

There are a number of problems with implementing static routes network-wide. Some include:

failure to scale well

does not automatically react/recover to changes in the network

tedious to configure for large networks (see point 1)

To configure a static route:

R1(conf)# ip route prefix mask address|interface [distance]

The prefix and mask is the destination network and subnet mask. You can use address to define the IP address of the next hop towards

the destination network or specify a local router interface that the router will use to send traffic out to the destination network. The

optional distance descriptive can be used to manually define the administrative distance for the route.

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Static Default Routes

One of the most common uses of static routes is for creating a default route. There are often cases when you want to forward packets that

is not defined in a specific route out an interface or towards another router. A common example is when connecting to an ISP. If traffic is

destined for an address range not defined within your organization (i.e. your coworker‘s Facebook updates), then it makes sense to

configure a default route towards your ISP or other organization.

To configure a static default route:

R1(conf)# ip route 0.0.0.0 0.0.0.0 address|interface

Floating Static Routes

There are some circumstances when it makes sense to use a static route as a backup to a dynamic routing protocol. In order for this to

work, however, the default administrative distance value on the static route must be raised so it will have a lower priority than the dynamic

routing protocol (see administrative distance section below).

Dynamic Routing

Dynamic routing protocols can dynamically respond to changes in the network. The routing protocol is configured on each router and the

routers learn about both each other and remote networks.

Examples of modern dynamic routing protocols include:

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RIP v1,2 (ok, maybe this isn‘t very ―modern‖)

EIGRP

IS-IS

OSPF

BGP

Distance Vector vs. Link-State

Distance Vector

When routers run a distance vector dynamic routing protocol, they periodically send information about their known routes to their

connected neighbors. This is how the router knows whether changes have been made to the network. They compare their routing table

against the information they receive from their neighbors – if it matches, their good. If not, they update their routing tables to reflect the

changes.

RIP is an example of a distance vector routing protocol.

Link State

Link state routing protocols operate differently. Routers send information about the state of their links to the entire network (or area) that

they are a part of. In this way, each router understands the entire network topology and must run an algorithm every time a network

change is announced to recalculate the best routes throughout the network. This makes link state routing protocols much more processor

intensive.

The second major difference in link state routing protocols is that updates are only sent is a change on a router‘s link occurs. This helps

keep bandwidth utilization low, unlike distance vector protocols which send out reoccurring updates regardless if a change has occurred.

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OSPF and IS-IS are examples of link state routing protocols.

Advanced Distance Vector

This is the tile Cisco gives to EIGRP, which borrows the best attributes of both distance vector and link state designs. EIGRP does not send

periodic route information, instead it sends updates only when changes occur (like link state protocols). Also, EIGRP forms neighbor

relationships with its directly connected peers and only updates them – not the entire network (like distance vector protocols).

Classful Concepts

IP routing protocols are either classful or classless and that determines how they present route information.

Classful

Classful routing protocols (like RIPv1) do not include the subnet mask in routing updates. When an update is sent, the packet contains only

the major network information depending on whether it is a class A,B, or C address.

For example, a route to network 172.16.10.0/24 would be advertised as 172.16.0.0/16 because its classful boundary is a class B address.

Obviously if you have broken your major network boundaries up into smaller subnets that are more granular than the major classful

boundaries, this will not work well and that‘s the reason almost all modern routing protocols are classless.

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Classeless

Classless routing protocols (like RIPv2, EIGRP, OSPF, IS-IS, and BGP) include the subnet mask in routing updates allowing for VLSM support

and supernetting.

Administrative Distance

Routers need a way of determining which path to use to a destination network if two or more routing protocols are in use and both

advertise a route. Administrative distance is Cisco‘s answer. Cisco has assigned an administrative distance (AD) to each routing protocol

that outlines which protocol a router will prefer. The AD values can be between 0 and 255 with the lowest values being used for routing.

Default AD values :

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For example, if router R1 receives a route to network 10.10.10.200.0 from both EIGRP and a OSPF, the router will compare the

administrative distance of the EIGRP learned route (90), to that of OSPF (110). The router will then add EIGRP‘s route to the routing table

because its AD is lower (90 < 110).

Summary

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E GRP Enhanced Interior Gateway Routing Protocol

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Enhanced Interior Gateway Routing Protocol, or EIGRP, is a Cisco proprietary, advanced distance vector dynamic routing protocol.

EIGRP Characteristics

Fast Convergence

EIGRP uses the DUAL algorithm to converge very quickly. It does this by knowing neighbor router‘s routing tables and predefining primary

and secondary routes to every destination network.

Triggered Updates

EIGRP uses partial triggered updates to its directly connected neighbors rather than periodically sharing its entire routing table. This saves

link bandwidth because updates are only sent if a change is incurred, only the changes are sent in the update, and lastly – the updates are

only sent to a routers‘s affected neighbors. Very efficient!

Protocol Independent

Enhanced Interior Gateway Routing Protocol supports more than just IPv4. It supports IPv4, IPv6, IPX, and AppleTalk.

Multicast

EIGRP sends route updates, hellos, and queries to its neighbors using the multicast address 224.0.0.10 so end hosts are not affected.

Hellos are sent out every 5 seconds by default to learn about new neighbors and make sure existing neighbors are still available.

VLSM

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Variable length subnet masking is supported by EIGRP because it is a classless routing protocol. That means subnet masks are included in

route updates.

Terminology

Feasible and advertised distance

EIGRP‘s DUAL algorithm determines the best route to a particular network by using distance information, known as cost or metric. DUAL

determines the lowest cost path by adding up the cost to the destination network. Neighbors exchange the cost to every route they know

of when a neighbor adjacency is formed. A router then uses that information to calculate their own cost to the same network by adding the

cost between themselves and their neighbor, then adding that to the neighbor‘s advertised cost.

So, (the cost between neighbors) + (the neighbor‘s cost to the destination network) = the total cost to the network, or the feasible

distance. The cost the neighbor advertised to the remote network is know as the advertised distance.

See the diagram below.

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Successor

Think of the successor as the active, or primary, route to a destination for EIGRP. The successor is actually the neighbor router that has the

least-cost path to a destination network (a.k.a. has the lowest feasible distance). Successor routes are added directly to the routing table.

You should also know that if multiple successors can exists if they have identical feasible distance values.

Feasible Successor

This is more like the backup route EIGRP chooses to a destination network. This is what makes EIGRP convergence so unique and so fast.

It always tries to find a backup route to that in the even that the successor fails, it can immediately switch over to the feasible successor

(backup) route with very little delay. To qualify as a feasible successor, the AD must be less than the successor‘s FD. This helps ensure a

loop-free layer 3 path.

Tables

Neighbor Table

EIGRP discovers neighbors by sending out hellos every 5 seconds. When a routers receives a hello with the same AS number defined, it

forms an adjacency and adds the local interface it used to reach it as well as the neighbor‘s IP address to the EIGRP neighbor table.

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Topology Table

When routers form an adjacency, they exchange route information. That

information is transferred to the EIGRP topology table, which contains all

the destinations advertised by a router‘s neighbors.

There are two different types of entries in the topology table, active and

passive. Now you may think that the active entry is the preferred or

―actively-in-use‖ route, but surprisingly, the opposite is true. The route in

the topology table that is in the active state signifies that it is ―actively‖

looking for an alternative path to a destination because the successor has

failed and no FS exists. Obviously this is not an ideal scenario.

If a router‘s successor becomes unavailable, but has a feasible successor –

the FS will immediately become the successor and there is almost no delay

incurred. This is the primary reason EIGRP convergence times tend to be

some of the fastest of all the dynamic routing protocols.

If, however, a router‘s successor becomes unavailable and does not have a

FS to the destination, it will send query messages to all of its neighbors

asking if they know of a path to the destination. The neighbors will either

respond with a path or forward the query to all of their neighbor routers

until a path is identified and relayed back to the original requester or no

more neighbor routers exist. During the time the router is waiting back for

a response, it is unable to forward traffic to the destination network, which

can hurt EIGRP‘s convergence time.

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Passive entries represent routes that have at least a single successor and perhaps a feasible successor. They are what you should see in a

normal, stable topology. Notice the ―P‘s‖ in the output from the show eigrp topology command below. They indicate that the entries in the

EIGRP topology table are in the passive (read: normal) state.

——————————————–

R1#sh ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(10.1.1.1)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply, r – reply

Status, s – sia Status

P 10.1.3.0/24, 1 successors, FD is 156160

via 10.1.100.3 (156160/128256), FastEthernet0/0



via 10.1.200.2 (2297856/128256), Serial1/0


via Connected, Loopback1




via Connected, FastEthernet0/0


via Connected, Serial1/0

——————————————–

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EIGRP Messages

Hello

EIGRP hello packets are sent out every 5 seconds by default using multicast address 224.0.0.10 to maintain and discover neighbor

relationships. On slower (T1 and below) and NBMA links, hellos are sent every 60 seconds to conserve bandwidth.

EIGRP hello packets also contains a hold timer which lets the router know if a neighbor is down. The hold timer is set to 15 seconds

normally (~3 unresponsive hellos), and 180 seconds for slower WAN links. When a router receives a hellos packet from another router with

the same AS (Autonomous System) number, it automatically forms a neighbor relationship (also known as an adjacency).

Update

During the EIGRP start-up process on a router, an update message is sent out to its neighbors containing the contents of the router‘s

routing table. The only other time an update packet is sent is when network changes occur on a router and it then sends out an update

message to its neighbors who the route change would affect.

Query

When EIGRP looses its successor route and does not have a FS, it sends out a query message to all of its neighbors asking if they know a

path. (See topology section above)

Ack

Acknowledgement packets are sent in response to update, query, and reply packets.

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Reply

When a router responds to a neighbor router looking for a route (query), it sends it in the form of a reply.

Graceful Shutdown

When an EIGRP process is shut down, the router sends out ―goodbye‖ messages to its neighbors (ironically in the form of hello packets).

The neighbors can then immediately begin recalculating paths to destinations that went through the shutdown router without having to wait

for the hold timer to expire.

EIGRP Metrics

There are 5 descriptives EIGRP uses to calculate its metric, although Cisco generally does not recommend tuning these metrics unless you

have a very specific purpose. You should be aware that only the bandwidth and delay numbers factor into the default formula.

Bandwidth – the lowest bandwidth value between the source and destination

Delay – the cumulative delay along a series of links

Reliability

Load

MTU

EIGRP Configuration

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Step 1.

Define EIGRP as the routing protocol with a predefined Autonomous System ID. Routers will not form a neighbor relationship if their AS

numbers do not match.

Example:

R3(config)# router eigrp 1

Step 2.

Define the attached networks you want to participate in EIGRP

Add each network to the EIGRP process with the network prefix mask command for each network. The mask is an inverted mask, like ACLs

use. Example, a /24 mask would be 0.0.0.255.

The network prefix mask command tells the router which local interfaces will then participate in EIGRP. This can be very useful if you do

not want specific interfaces to participate in EIGRP.

Using the mask statement will define how you want the routes summarized if you turn off auto summarization. If you choose not to use the

mask, EIGRP will assume the networks are part of the major networks (class A,B,C boundaries) and could cause potential problems.

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Example:

R3(config-router)#router eigrp 1

R3(config-router)# network 10.1.100.0 0.0.0.225


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R3(config-router)# no auto-summary

The output of R3′s running configuration can be seen below.

R3#sh run | begin router eigrp 1

router eigrp 1

network 10.0.0.0

network 192.168.100.0 0.0.0.3

network 192.168.100.4 0.0.0.3

no auto-summary

!

EIGRP Verification

show ip eigrp neighbors

Displays EIGRP neighbors a router has discovered.

——————————————–

R3#sh ip eigrp neighbors

IP-EIGRP neighbors for process 1

H Address Interface Hold Uptime SRTT RTO Q Seq

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(sec) (ms) Cnt Num

1 10.1.100.2 Fa0/0 13 00:12:23 737 4422 0 21

0 10.1.100.1 Fa0/0 14 00:12:29 535 3210 0 22

show ip eigrp topology

Displays the output of the EIGRP topology tables including successor and feasible successor routes.

——————————————–

R3#sh ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(192.168.100.5)

Codes: P – Passive, A – Active, U – Update, Q – Query, R – Reply,

r – reply Status, s – sia Status











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via Connected, FastEthernet0/0




show ip route

Shows the ip routing table entries for all routing protocols.

——————————————–

R3#sh ip route

Codes: C – connected, S – static, R – RIP, M – mobile, B – BGP

D – EIGRP, EX – EIGRP external, O – OSPF, IA – OSPF inter area

N1 – OSPF NSSA external type 1, N2 – OSPF NSSA external type 2

E1 – OSPF external type 1, E2 – OSPF external type 2

i – IS-IS, su – IS-IS summary, L1 – IS-IS level-1, L2 – IS-IS level-2

ia – IS-IS inter area, * – candidate default, U – per-user static route

o – ODR, P – periodic downloaded static route

Gateway of last resort is not set

10.0.0.0/24 is subnetted, 5 subnets

C 10.1.3.0 is directly connected, Loopback3

D 10.1.2.0 [90/156160] via 10.1.100.2, 00:14:46, FastEthernet0/0


C 10.1.100.0 is directly connected, FastEthernet0/0

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[90/2172416] via 10.1.100.1, 00:14:46, FastEthernet0/0




show ip route eigrp

Displays the EIGRP routes that the routing table is using. All internal EIGRP routes will be marked with a D (as in DUAL) at the beginning.

——————————————–

R3#sh ip route eigrp





[90/2172416] via 10.1.100.1, 00:16:49, FastEthernet0/0

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Additional EIGRP configuration options

EIGRP Default Routes

Defaults routes make life easier in many situations. They can decrease the size (and complexity) of the routing table by providing a path to

all unspecified destinations.

One option is to use a static default route with the ip route 0.0.0.0 0.0.0.0 interface/address statement as discussed in the Routing

Fundamentals page. This must be configured on every router that will use that default route.

Another option if you are running EIGRP is to use the ip default-network network-number command IN GLOBAL CONFIG MODE. Any

network that is reachable within the local router‘s routing table is eligible to be used by EIGRP as a default route. Once configured, EIGRP

will advertise the route to its EIGRP neighbors as a default route.

** If you want to use this method, in conjunction with a static route – you will have to first redistribute the static route into EIGRP.

** Once you use the IP default-network command to define a default route for EIGRP, the router creates a static route in the configuration

without notifying you. That means in order to remove the default route, you must use the no ip route command instead of no ip default-

network.

Summarization

EIGRP summarizes routes by their major classful boundaries, which can be problematic and cause specific subnets to not be advertised

correctly.

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To disable automatic summarization:


R1(config-router)# no auto-summary

It is also possible to manually summarize routes with EIGRP out specific interfaces. Under the interface configuration mode, use the ip

summary-address eigrp autonomous-system command.

R1(config)# intferface s0/0/0

R1(config-if)# ip summary-address eigrp 1 10.1.2.0 255.255.255.0

EIGRP over WAN Networks

EIGRP + MPLS

MPLS defines the customer‘s WAN routers as CE, or customer edge routers and the carrier‘s border routers as PE, or provider‘s edge

routers. The CE routers appear to each other as directly connected peers. When CE West sends information to CE East, PE West intercepts

the data, strips the Ethernet frame, encapsulates it into a MPLS packet, and forwards it over the service provider‘s network to PE East. PE

East strips off the MPLS information, re-encapsulates it into an Ethernet frame and forwards it on to CE East.

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This transparent transport allows an EIGRP neighbor relationship to form between the two customer routers.

EIGRP + Frame Relay

Let‘s face it, frame relay is a dying WAN technology. Other, more current WAN options like MPLS have taken over, but Cisco thinks it‘s

important for us to understand the underlying framework of how frame relay works. Frame relay works using switched, virtual circuits

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through the service provider network. One of the advantages of Frame Relay is that it allows multiple logical circuits to be configured on a

single physical interface. Each VC is identified with a locally-significant DLCI, or Data-Link Connection Identifier. The layer 2 virtual circuit

must then be mapped to a layer three neighbor, which can be either dynamic or static.

Frame relay is able to emulate point-to-point links by using multiple subinterface on a single physical interface (often used on hub-and-

spoke topologies). This allows neighbor‘s to be identified as down much more quickly for two reasons:

1. The default timers are shorter (5 sec hold timer, 15 second dead timer).

2. The subinterface is marked down whenever its local DLCI goes down.

Static

To configure frame relay statically, configurations must be done on the interface level. The broadcast descriptive is required at the end of

the statement because frame relay defaults to a non-broadcast medium. Also, static mappings can be applied to both multipoint interfaces

as well as subinterfaces on a single physical port.

R1(config-if)# frame-relay map ip remote-ip-address loacl-dlci broadcast

Dynamic

Dynamic mappings use inverse ARP. In this case, routers only form EIGRP neighbor relationships with other routers they connect to using a

frame relay virtual circuit.

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No IP split horizon

When running EIGRP on a frame relay multipoint subinterfaces, a major communication problem can occur. Split-horizon is a method of

preventing routing loops in distance-vector routing protocols by prohibiting a router from advertising a route back onto the interface from

which it was learned.

When a hub and spoke frame relay topology exists, multipoint subinterfaces are configured on the hub router. The issue is that split

horizon is enabled by default, so in the example below, if R2 learns routes from R1, it cannot then pass those on to R3 because split horizon

would prevent the advertisement from going out the same physical interface. This results in R2 being able to communicate with the spoke

router‘s networks, but R3 and R1 are unable to communicate with each other.

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To remedy the situation, split horizon must be disabled on the R2 EIGRP process.

R2(config-if)# no ip split-horizon EIGRP as-number

Managing EIGRP Bandwidth

There are two important points to remember when running EIGRP over WAN links. The first is that EIGRP assumes that WAN interfaces run

at T1 speed (1544 kbs). The second is that EIGRP will allocate up to 50% of a link‘s bandwidth for EIGRP control traffic.

These two combined can be problematic on links that are slower than a T1 (like a 64k fractional T1 for example). In that situation, EIGRP

messages could choke out data traffic quickly. To control that, the bandwidth command should be used in WAN links to tell EIGRP what the

actual link bandwidth is.

R1(config)# int serial 0/0/0

R1(config-if)# bandwidth 64

EIGRP is often used on frame relay for this reason alone. The ability to control the routing protocol‘s usable bandwidth so simply makes it a

popular choice.

More EIGRP options

Passive Interfaces

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Not to be confused with the passive (healthy) topology table entries, interfaces with the passive-interface command applied do not allow

any routing updates or hellos out the interface. For EIGRP, this means that the router will not form adjacencies with connected routers on

that particular port.


R1(config-router)# passive-interface gig 3/1

Unicast

EIGRP uses multicast address 224.0.0.10 when sending messages to its neighbors. You should be aware that EIGRP can also use a unicast

address when communicating with a specific neighbor. To configure it:

R1(config)#router eigrp 1

R1(config-router)# neighbor ip-address

The IP address used must be in one of the same subnet ranges as one of the router‘s interfaces.

EIGRP load balancing

Out of the box, EIGRP will automatically load balance across equal-cost paths with no special configuration. EIGRP is unique, however, in

its ability to load balance across unequal-cost paths with a single command.

The variance command allows unequal-cost load balancing over up to 6 different paths. But here‘s the key, it only works when the cost of

the path is lower than the variance number multiplied by the best metric.

Here is an example scenario.

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R1 will by default use the path through R3 because it has the lowest metric. To enable unequal-cost load balancing, we can use the

following command:


R1(config-router)# variance 2

The variance command multiplies the best cost (10,000) by 2 (20,000) and will begin load balancing across all paths with a FD less than

that – which includes the path through R2(15,000). The will load balance the traffic in proportion to each path‘s metric.

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Maximum-paths

By default, Cisco IOS will load balance across 4 equal-cost paths only. Using the maximum-paths command, you can configure the router

to load balance over up to 16 paths. Setting it to 1 disables the load balancing.

R1(config)# maximum-paths number-of paths

EIGRP Authentication

EIGRP supports authentication of its messages using an MD5 hash. When configured, if an incoming EIGRP packet‘s hash does not match

the local hash, the packet is silently dropped.

Authentication configuration steps:

1. Configure a key chain to group the keys (read: passwords).

2. Create a key(s) inside the keychain. The router will look inside the keychain and compare the keys against incoming packets.

3. Enable authentication and assign a key to an interface,

4. Indicate MD5 as the authentication type.

Example

R1(config)# key chain TEST

R1(config-keychain)# key 1

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R1(config-keychain-key)# key-string samplepassword

R1(config-keychain-key)# exit

R1(config)# interface gig 1/12

R1(config-if)# ip authentication mode eigrp 10 md5

R1(config-if)# ip authentication key-chain eigrp 10 TEST

EIGRP Stub Routing

If a router is a spoke in a hub-and-spoke router topology, it is considered a stub router. It is not a transit router and usually has only a

single neighbor router, sometimes two.

Within EIGRP you can define a router as a stub router to limit the EIGRP queries. This saves bandwidth and prevents neighbor routers from

requesting alternate routes when a path fails. If you have many spoke routers, this can dramatically improve EIGRP reconvergence time.

The EIGRP stub router still receives all route updates from its neighbor(s) by default.


R1(config-router)# eigrp stub [receive-only | connected | static | summary | redistributed]

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EIGRP Best Practices

Summarize routes when possible.

Limit the network depth to 7 hops.

Limit the scope of EIGRP queries

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SPF Open Shortest Path First

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OSPF, or Open Shortest Path First, is a link-state, open-standard, dynamic routing protocol. OSPF uses an algorithm known as SPF, or

Dijkstra‘s Shortest Path First, to compute internally the best path to any given route.

OSPF is classless and converges fairly quickly, using cost as it‘s metric. A router running OSPF creates its own database which contains

information on the entire OSPF network, not simply neighbor‘s routes like EIGRP. This allows the router to make intelligent choices about

path selection on its own instead of relying exclusively on neighbor information.

OSPF routers do form neighbor relationships though. They exchange hellos with neighboring routers and in the process learn their

neighbor‘s Router ID (RID) and cost. Those values are then sent to the adjacency table.

Every router is responsible for computing their own best paths to all destinations within an OSPF domain. Once the SPF algorithm selects

the best paths, they are then eligible to be added to the routing table.

Link State Database

Once a router has exchanged hellos with its neighbors and captured Router IDs and cost information, it begins sending LSAs, or Link State

Advertisements. LSAs contain the RID and costs to the router‘s neighbors. LSAs are shared with every other router in the OSPF domain. A

router stores all of its LSA information (including info it receives from incoming LSAs) in the Link State Database (LSDB).

I apologize if the acronyms are starting to pile up. OSPF, architecturally speaking, is more complicated than it‘s counterpart EIGRP – and

the long list of acronyms and definitions is part of that.

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Areas

OSPF is different from EIGRP in that it uses areas to segment routing domains. This helps partition routers into manageable groups if the

layer 3 network begins to get large.

It all starts with area 0. Every OSPF network must contain an area 0, sometimes referred to as the backbone area and every additional area

must be physically connected to area 0. From there, other areas are optional.

Note that the SPF algorithm only runs within a single area, so routers only compute paths within their own area. Inter-area routes are

passed using border routers.

All link state databases must match within an OSPF area.

This means that the more OSPF-enabled routers are

configured for the same area, the more LSA

advertisements that must be sent out. After you reach

about 50 routers, the high levels of LSA traffic and

numerous routing table entries can become a problem.

That is why Cisco recommends limiting an OSPF area to

no more than 50-100 routers.

The following three factors determine the

maximum number of routers:

How easily the area‘s subnets can be summarized

The type of areas being used

The number of external LSAs being injected

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An added bonus of partitioning out your OSPF network into areas is that it is a natural fit for a hierarchical IP scheme.

Area Types

Backbone area

Another name for area 0

Regular area

Non-backbone area, with both internal and external routes

Stub area

Contains only internal routes and a default route

Totally Stubby Area

Cisco proprietary option for a stub area

Not-So-Stubby area (NSSA)

Contains internal routes, redistributed routes, and optionally a default route

Totally Stubby NSSA

Cisco proprietary option for NSSA

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Router Roles

Internal: All interfaces in a single area (routers 1,4,5 in diagram above)

Backbone: At least one interface assigned to area 0 (routers 2,3 in diagram above)

Area Border Router (ABR): Have interfaces in two or more areas (routers 1,4,5 in diagram above)

ABRs contain a separate Link State Database, separating LSA flooding between areas, optionally summarizing routes, and optionally

sourcing default routes.

Autonomous System Boundary Router (ASBR): Has at least one interface in an OSPF area and at least one interface outside of an

OSPF area.

OSPF Metric

Each interface is assigned a cost value based purely on bandwidth. The formula is:

Cost = (100Mbs/bandwidth)

Higher bandwidth means a lower cost!

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Let‘s run through some common examples quickly:

T1 line | 100,000 / 1544 = 64

10 Mbps | 100,000 / 10,000 = 10

100 Mbps | 100,000 / 100,000 = 1

1000 Mbps | 100,000 / 1,000,000 = .1 1 (OSPF still uses 1 for this, see explanation below)

The cost is then accrued at each hop along the path based on the link‘s bandwidth. Unfortunately, OSFP was written when 100Mbs was

considered fast. Because of that, it assigns the same cost to any interface with speeds higher than 100Mbs. To OSPF, a Fast Ethernet

interface is weighted the same as a Gigabit Ethernet interface, both a cost of 1.

To fix that problem, you can use the auto-cost command under the OSPF process.

R1(config-router)# auto-cost reference-bandwidth 1000

Another option is to simply change the cost on a per-interface basis with the ip ospf cost command (using any number between 1-

65,535).

R1(config-if)# ip ospf cost 35

Link State Advertisements

LSAs contain a sequence number and a Router ID. Sequence numbers are 32 bits, starting with 0×80000001.

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The sequence number increases if:

a route is added or deleted

a LSA ages out

The largest sequence number is always the most current. The default time that LSAs are aged out is 30 minutes. When an LSA enters a

router, it checks it against its internal Link State Database (LSDB).

If it is new, it is added to the LSDB and the SPF algorithm is re-run.

If it contains a Router ID (RID) that is already in the database, entries with an older sequence number are discarded.

If it receives an older version (according to its sequence number), it discards the LSA and sends back the newer version to the

original sender.

The command show ip ospf database will display the sequence numbers and age (in seconds) for each entry.

LSDB Overload

In large OSPF networks, if major network changes occur, a flood of SLAs will immediately hit the entire network. The number of incoming

LSAs to each router could be substantial and bring the CPU and memory to its knees.

To mitigate that scenario, Cisco offers what it refers to as Link Sate Database Overload Protection. Once enabled, if the defined threshold

is exceeded over one-minute time period, the router will enter the ignore state – dropping all adjacencies and clearing the OSPF database.

Know that this is a drastic response because routing will disrupted during that period.

R1(config-router)# max-lsa number

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LSA Definitions

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OSPF Messaging

OSPF uses several different types of messages to maintain neighbor relationships and correct routing information.

OSPF Packet Types

Hello

Discovers neighbors and works as a keepalive.

Link State Request (LSR)

Requests a Link State Update (LSU), see below.

Database Description (DBD)

Contains a summary of the LSDB, including RIDs and sequence numbers.

Link State Update (LSU)

Contains one or more complete LSAs.

Link State Acknowledgement (LSAck)

Acknowledges all other OSPF packets (except hellos).

OSPF sends the five packet types listed above over IP directly, using IP port 89 with an OSPF packet header. Multicast address 224.0.0.5 is

used if sending to all routers, address 224.0.0.6 is used for sending to all OSPF DRs.

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OSPF Neighbors

Hellos are sent out periodically using multicast on OSPF enabled routers. The router forms an adjacency with a peer router when it sees its

own Router ID in the neighbor field of another router‘s hello message. That indicates there is direct, bi-directional communication on the

same subnet.

Note: On multi-access links, adjacencies are only formed between the router and the DR and BDR.

All of the following fields in an OSPF hello message must match for an adjacency to form:

hello timer

dead timer

area ID

authentication type

password

stub area flag

As with many network protocols, hellos act as a form of keepalive or heartbeat. With OSPF, if four consecutive hellos are not received (the

dead time), the router is considered down.

Point-point interfaces: hellos every 10 seconds, 40 second dead timer

Nonbroadcast multiaccess (NBMA) interfaces: hellos every 30 seconds, 120 second dead timer

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OSPF States

There are 7 different OSPF states when forming neighbor relationships. Take the time to learn what states provide which functions.

1. Down State

OSPF has not started and no hellos have been sent.

2. Init State

Hellos are sent out all OSPF-participating interfaces

3. Two-way State

A hello is received from another router with its own RID in the neighbor field. All other required elements match and the routers become

neighbors.

4. Exstart State

Routers determine which one will begin the route exchange process with the other.

5. Exchange State

Routers exchange DBDs.

6. Loading State

Routers compare the DBD to their LS database. LSrs are sent out for missing or outdated LSAs. Each router then responds to the LSRs

with a Link State Update. Finally, the LSUs are acknowledged.

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7. Full State

The LSDB is completely synchronized with the OSPF neighbor.

OSPF Configuration

OSPF configuration is not too complicated, but has some important syntax distinctions from EIGRP. First, it is configured from router

configuration mode and requires a process ID appended to the router ospf command. The process ID is only locally significant, so don‘t

worry if it doesn‘t match on other OSPF routers.

R1(config)# router ospf process-id

The next step is to determine which router interfaces you want participating in OSPF. Just like EIGRP, the network statements define which

local router interfaces will participate.

R1(config)# router ospf 10

R1(config-router)# network 10.1.1.0 0.0.0.255 area 0


In the example above, interfaces in the 10.1.1.0/24 subnet will participate in OSPF area 0. Interfaces in the 10.9.9.0/24 subnet will

participate in OSPF area 1. Unlike EIGRP, the subnet wildcard mask in the network statement is not optional because OSPF is classless by

default.

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Let‘s do another example…

R1 has six interfaces, all within area 0:

GigabitEthernet 0/0: 192.168.100.1/24




Serial 1/0: 10.100.100.1/30

Serial 1/1: 10.100.100.5/30

The simplest way to configure OSPF an all interfaces into area 0 would be to use this command:


A second option is to break up the 10. and 192. networks into different statements:



The third way to configure the interfaces to participate in OSPF:







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All three approaches achieve the exact same result. The configuration you choose is up to you.

Interface Configuration

An alternative configuration option is to configure an interface to participate in OSPF directly. The [ ip ospf process-id area area-id ]

command takes precedence over the more common network commands.

R1(config)# int gig 0/1

R1(config-if)# ip ospf 10 area 0

Router ID

The SPF algorithm uses a Router ID to identify hops along a path. The problem, of course, is that routers don‘t have a generic ―router ID‖

lying around.

The designers of OSPF decided to use the highest IP address assigned to a loopback interface as the Router ID (RID) by default. If no

loopback is configured, it will use the highest IP address assigned to an active interface when the OSPF process begins. OSPF will not

change the RID, even if another interface with a higher IP address comes online unless the OSPF process is restarted. This helps keep the

network stable and happy. Note: The clear ip ospf process command will also force the OSPF process to restart, but will cause an outage

– so use it with caution.

Loopbacks are preferred for use as a router ID because they are virtual interfaces and are not affected by links going up and down. To

configure a loopback interface, first create it and assign it an IP address.

R1(config)# int loopback 0

R1(config-if)# ip address 10.100.100.1 255.255.255.255

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Static RIDs

It is also possible to manually define a static Router ID within OSPF with the router-id command.


R1(config-router)# router-id 10.100.100.1

DRs & BDRs

SPF works by mapping all paths to every destination on each router. It uses the RID to identify hops along each path and uses bandwidth

as a metric between those hops. This whole system works really well when routers are connected with point-to-point links and OSPF traffic

is simply sent using multicast address 224.0.0.5.

It doesn‘t work well, however, when a router is connecting to multiaccess networks like an Ethernet VLAN. Multiaccess OSPF links require a

Designated Router (DR) be elected to represent the entire segment. Another router is then elected as the Backup Designated Router, or

BDR. On that specifc multiaccess segment, routers only form adjacencies with the DR and BDR.

The DR uses type 2, network LSAs to advertise the segment over multicast address 224.0.0.5. The Non-Designated routers then use IP

address 224.0.0.6 to communicate directly with the DR.

Elections

1. When the OSPF process on a router starts up, it listens for hellos. If it does not receive any within its dead time, it elects itself the DR.

2. If hellos are received before the dead time expires, the router with the highest OSPF priority is elected as the DR. Next, the same

process happens to elect the BDR.

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Note: If a router‘s OSPF priority is set to 0, it will not participate in the elections.

3. If two routers happen to have the same OSPF priority, the router with the highest Router ID will become DR. The same is true for BDR.

Once a DR is elected, elections cannot take place again until either the DR or BDR go down. This essentially means that there is no OSPF

DR preemption if another router comes online with a higher OSPF priority. In the case that the DR goes down, the BDR automatically is

assigned the DR role and a new BDR election occurs.

Be aware that a router with a non-zero priority that happens to boots first can become the DR just because it did not recieve any hellos

when the OSPF process was started – even though it may have a low OSPF priority.

The default OSPF priority is 1 and Cisco recommends manually changing that on routers you want to become the DR and BDR. Remember

that DRs are only used on multiaccess links, so they are only significant on an interface level. A router with two different interfaces

connected to two different multiaccess links will have separate DR elections for each segment.

To set the OPSF priority, use the ip ospf priority command on the interface connected to the multiaccess segment. Values can be

between 0-255.


R1(config-if)# ip ospf priority 255

OSPF over the WAN

Routing protocols assume both broadcast capabilities and full mesh connectivity on multiaccess networks. For OSPF, there are a few points

to consider:

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Full mesh environment can use physical interfaces, but often times subinterfaces are used

Partial mesh environments should be configured using point-to-point subinterfaces

Hub-and-spoke environments should elect the hub as the DR or use point-to-point subinterfaces – which don‘t require a DR

Frame Relay and ATM maps should include the broadcast attribute

In multiaccess environments, the DR and BDR should have full virtual circuit connectivity to all other routers

Summarization

First, it‘s important to note that running the SPF algorithm on a router is extremely taxing on CPU resources and can easily consume them

all. The reason is because OSPF has to compute the best path to every destination within its area. Avoiding running the alogrithm

whenever it isn‘t required is a big win.

Summarization has two importnat benefits for OSPF. It prevents toplogy changes from being passed outside an area – thus reducing the

number of routers re-running the SPF algorithm. It also consolidates many routes in to a single statement, reducing the memory load and

database size on OSPF-enabled routers.

There are two types of route sumarization, inter-area and external.

Inter-area Summarization (LSA Type 3)

This occurs on ABRs to summarize routes between areas. This really only works well if the networks contained within an area are subnetted

contiguously so that they can be easily summarized into a single statement.

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The new summary route‘s cost will be equal to the lowest cost route within the summary range. After the command is entered, the router

will automaticlly create a static route pointing to Null0.

Example:

ABR-R1(config)# router ospf 10

ABR-R1(config-router)# area 2 range 10.100.0.0 255.255.0.0

In this example, the summary network 10.100.0.0/16 is summarized from area 2.

External Summarization (LSA Type 5)

This occurs on ASBRs for routes that are injected into OSPF via route redistribution. After the command is entered, the router will

automatically create a static route pointing to Null0.

Example:

ASBR-R1(config)# router ospf 10

ASBR-R1(config-router)# summary-address 192.168.0.0 255.255.0.0

In this example, an external network has been summarized into 192.168.0.0/16 and is injected into OSPF via a single type 5 LSA.

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OSPF Passive Interfaces

Like EIGRP, OSPF supports the use of passive interfaces. The passive-interface interface command disables OSPF hellos from being sent

out, thus disabling the interface from forming adjacencies out that interface.

OSPF Default Routes

Default routes are injected into OSPF via type 5 LSAs. There are multiple ways to inject default routes into OSPF, but Cisco recommends

using the default-information originate command under the OSPF routing process.


R1(config-router)# default-information originate [always] [metric metric]

If the always keyword is not used, OSPF will advertise a default route learned from another source, like a static route. If the always

keyword is present, a default route will be advertised regardless if the route exists in the routing table.

Another option is to use the area range and summary-address commands discussed in the summarization section above. Using these will

result in the router advertising a default route pointing to itself.

Stub and Not-So-Stubby Areas

Stub areas are another way to simplify route information that gets advertised. Area 2 in the diagram above shows an example.

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The ABR in a stub area drops all

external routes and instead uses a

default route of 0.0.0.0 (R3 in this

example). That is, they do not know

about any non-OSPF route information

outside their own area.

A Cisco proprietary version of a stubby

area is a Totally Stubby Area, or TSA.

TSAs do not accept any external routes

from non-OSPF sources AND they do

not accept routes from other areas

within their OSPF autonomous system.

If a router needs to send traffic to a

route outside of its own area, it sends

the traffic using a default route.

*ABRs use default routes in Stub

and Totally Stubby areas.

Stubby areas are made into Totally Stubby Areas by appending the no-summary keyword.

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Example:


R5(config-router)# area 2 stub no-summary

R5(config-router)# area 2 stub default-cost 8

The example above sets area 2 as a totally stubby area. The default-cost command is optional and in this case changed the default route

cost from 1 to 8.

Stub Limitations

Virtual links cannot be included

Cannot include an ASBR

The stub configuration must be applied to every router within the stubby area

Area 0 cannot be a stub

Bullet point 3 is extremely important! If two routers are connected, but one does not have the stub statement configured, the hello packets

will be dropped and they will not form a neighbor adjacency.

Not-So-Stubby Areas, or NSSAs were an addendum to the original OSPF RFC and defined a new special LSA, type 7. NSSAs are very

similar to stubby areas, but they allow the use of ASBRs in the area – something stub areas prohibit.

External routes are advertised by the ASBR as type 7 LSAs and the ABR then converts them into type 5 external LSAs when it advertises

them to adjacent areas.

NSSA is configured using the area area-number nssa command as can been seen in the example below. Using the no-summary

keyword turns the area into a Totally Stubby NSSA. A Totally Stubby NSSA does not accept external or summary routes from other areas.

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Lastly, the NSSA ABR does not by default advertise a default route back into the area. The default-information-originate option does just

that.


R4(config-router)#area 1 nssa [no-summary] [default-information-originate]

The table below should help recap the different area type behaviors.

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OSPF Virtual Links

OSPF has strict rules around how areas connect and where they can be located. More specifically, every area must be physically connected

to area 0 and area zero must be ‗contiguous‘ – meaning it cannot broken into multiple, connected area 0s.

Virtual links were developed as a band-aid to situations that temporarily must violate those requirements. Virtual links connect areas that

do not connect directly to area 0. It can also connect two area 0s together! Keep in mind that Cisco recommends virtual links be a

temporary workaround to a short-term problem, not a permanent design.

The diagram below illustrates an example when a virtual link could be used. Let‘s pretend Company ABC and Company XYZ just announced

a merger and now their corporate networks must do the same. In this case, both routers R1 and R2 have now become ABRs and the virtual

link configuration will be applied to them.

The command area area-number virtual-link router-id is applied to each ABR. Note that the area used in the command is the transit area

that the virtual link resides in. Also, the RID identifies the RID of the OTHER router at the end of the link!

Example


R1(config-router)# area 1 virtual-link 10.30.30.30


R2(config-router)# area 1 virtual-link 10.50.50.50

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OSPF Authentication

Out of the box, OSPF does not authenticate its protocol‘s messages or route updates. OSPF does, however, support two message

authentication options:

Simple Authentication - using plaintext keys

MD5 Authentication

Matching authentication methods and keys must configured on each interface on a segment. Theoretically, different passwords could be

applied to different router interfaces – the routers on the other ends of those links would just be required to have matching information.

Simple Authentication Example

R1(config)# int fa0/1

R1(config-if)# ip ospf authentication-key KEY123

R1(config-if)# ip ospf authentication

R1(config-if)# exit


R1(config-router)# area 0 authentication

MD5 Authentication Example

R1(config)# int fa0/1

R1(config-if)# ip ospf message-digest-key 1 md5 KEY123

R1(config-if)# ip ospf authentication message-digest

R1(config-if)# exit


R1(config-router)# area 0 authentication message-digest

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** The 1 in theip ospf message-digest-key 1 md5 KEY123 statement above is a key number.

OSPF Verification

The OSPF neighbor table can be viewed using the show ip ospf neighbor command. It shows the status of the OSPF database loading

process, status of neighbor adjacencies, as well as DR and BDR assignments.

To show which OSPF routers are being used by the routing table, issue the show ip route ospf command.

The show ip ospf command displays the RID, counters, and timers.

To see which router interfaces are participating in OSPF (and their area assignments), use the show ip ospf interface command.

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ROUTE REDISTRIBUTION

& Filte

ring

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Redistribution is necessary when routing protocols connect and must pass routes between the two. This can happen in a number of

situations, but some examples include:

Organizations transitioning routing protocols

Businesses merge, and so must their networks

OSPF or EIGRP is used at the access and distribution layer of an enterprise and BGP is used in the core

The challenge to redistributing routing protocols is that each routing protocol uses it own metric and they are not compatible with each

other. Furthermore, there is no magic algorithm than can automatically translate metrics between, say RP and BGP.

To deal with this dilemma, a new seed metric is used as a staring point when redistribution is configured.

Configuring Redistribution

To configure redistribution between routing protocols, the redistribute protocol command is used under the routing protocol that recieves

the routes.

R1(config-router)# redistribute protocol [AS] [metric metric-vlaue]

The process-id field above is the AS number for RIP, EIGRP, BGP. For OSPF, use the process ID.

Also, both RIP and EIGRP require the use the metric keyword!

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EIGRP Redistribution Example:


R1(config-router)# redistribute ospf 20 metric 1000 100 255 1 1500

The example above shows OSPF being redistruted into EIGRP with a metric of 1000 100 255 1 1500. That is a lot of different numbers for

an EIGRP cost! That‘s because EIGRP redistribution metric requires you to input all of the metric calculation manually:

bandwidth

delay

reliability

loading

mtu

You can perform a show interface on the outgoing router interface prior to see what values the router is currently using.

OSPF Redistribution Example:


R1(config-router)# redistribute eigrp 10 subnets

The example above redistributes EIGRP routes into OSPF. The subnets keyword at the end of the redistribute command is extremely

important! Without this keyword, OSPF will redistribute networks at their classful boundaries – not something most administrators want.

If you don‘t use it the IOS will even give you a warning. Make sure to include it.

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Distribute Lists

Distribute lists are access lists applied to the routing process, determining which networks are allowed into the routing table or included in

updates. They essentially act as a filter.

Think: access list applied to routing = distribute lists

When creating a distribute list, use the following steps:

Step 1.

Identify the network addresses to be filtered and create an ACL – permitting the networks you want to be advertised.

Step 2.

Determine if you want to filter updates coming into the router or leaving the router.

Step 3.

Assign the ACL using the distribute-list command.

Incoming Distribute Lists:

R1(config-router)# distribute-list {acl-number | name} in [interface-type number]

Outgoing Distrubute Lists:

R1(config-router)# distribute-list {acl-number | name} out [interface-name | routing-process | AS-number]

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Route Maps

When a routing update arrives at an interface, a series of steps occur to process

it correctly. The diagram below outlines those steps and serves as a foundation

for the rest of this route redistribution and filtering section.

Route maps are extremely flexible and are used in a number routing scenarios

including:

Controlling redistribution based on permit/deny statements

Defining policies in policy-based routing (PBR)

Add more mature decision making to NAT decisions than simply

using static translations

When implementing BGP PBR

Route maps allow an administrator to define specific traffic and then take

automated actions against it to control how routing information is processed and

forwarded. Route maps uses logic similar to if/then statements in simple

scripting.

In route map terms, it matches traffic against conditions and sets options for

that traffic.

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NOTE: If you have downloaded the Switch Exam Guide, you will notice the similarity between the syntax structure of route maps and

VACLs.

Each statement in a route map has a sequence number, which are read from lowest to highest. The router stops reading statements when

it reaches its first matching statement.

Understand that there is an implicit deny included in all route maps. If traffic does not match any statement, it is denied.

Basic Route Map Configuration

R1(config)# route-map {tag} permit | deny [sequence_number]

That is how all route maps begin. Permit means that any traffic matching the match statement that follows is processed by the route map.

Deny means that any traffic matching the match statement that follows is NOT processed by the route map. Know the difference.

Match & Set Conditions

If no match condition exists, the statement matches anything (similar to a ‗permit any‘).

If no set condition exists, the statement is simply permitted or denied with no additional changes made.

If multiple match conditions are used on the same line, it is interpreted as a logical OR. In other words, if one condition is true, a match is

made. For example, the router would interpret ‗match a b c‘ as ‗a or b or c‘.

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If multiple match conditions are used on consecutive lines, it is interpreted as a logical AND. In other words, all conditions must be true

before a match is made. For example, the router would interpret the following example as match a and b and c:

route-map EXAMPLE permit 5

match a

match b

match c

Important route redistribution match conditions

ip address

Refers to an access list that permits or denies networks

ip address prefix-list

Refers to a prefix list that permits or denies prefixes

ip next-hop

Refers to an access list that permits or denies ip next hops IP

addresses

ip route-source

Refers to an access list that permits or denies advertising router

IP addresses

length

Permits or denies packets based on length (in bytes)

metric

Permits or denies routes with specific metrics from being

redistributed

route-type

Permits or denies redistribution based on the route type listed

tag

Routes can be labeled with a number that identifies it

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Important route redistribution set conditions

Metric

Sets the metric for redistributed routes

Tag

Tags a route with a numbered identifier

Route Map Verification

Use the show route-map command to verify route maps and PBR entries are filtering as expe

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BGP Border Gateway Protocol

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BGP, or Border Gateway Protocol is an external, dynamic routing protocol. It is most often used between ISPs and between enterprises and

their service providers. BGP is literally the routing protocol of the Internet because it connects independent network together, enabling end-

to-end transport. Scalability and stability are BGP‘s focus, not speed – as a result it behaves very differently than most other routing

protocols.

BGP is recommended whenever multihoming is a requirement (dual ISP connections to different carriers), when route path manipulation is

needed, and in transit Autonomous Systems.

A Quick Overview

Routers running BGP are called BGP speakers.

BGP uses autonomous system numbers to keep track of different administrative domains. 1-64511 are public, 64512-65535 are private.

BGP is used to connect IGPs, interior gateway protocols like OSPF and EIGRP. Routing between Autonomous Systems is referred to as

interdomain routing.

The administrative distance for eBGP routes is 20, iBGP is 200.

BGP neighbors are called ―peers‖ and must be statically assigned.

Peers receive incremental, triggered updates as well as keepalives using TCP port 179.

BGP is sometimes referred to as a ―path-vector‖ protocol because its route to a network uses AS numbers on the path to the destination.

BGP uses it‘s path-vector attributes to help in loop prevention. When an update leaves an AS, the AS number is prepended to update along

with all the other AS numbers that have spread the update. When a BGP router receives an update, it first scans through the list of AS

numbers. If it sees it own AS number, the update is discarded.

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BGP Databases

Like most modern routing protocols, BGP has two separate databases – a neighbor database and a BGP-specific database.

Neighbor Database

Lists all of the configured BGP neighbors (to view – #show ip bgp summary).

BGP Database

Lists all networks known by BGP along with their attributes. (to view – #show ip bgp).

BGP Message Types

There are four different BGP message types.

Open

After a BGP neighbor is configured, the router sends an open

message to establish peering with the neighbor.

Update

The type of message used to transfer routing information between

peers.

Keepalive

BGP peers sends keepalive messages every 60 seconds by default

to maintain active neighbor status.

Notification

If a problem occurs and a BGP peer connection must be dropped,

a notification message is sent and the session is closed.

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Internal vs. External

iBGP, or internal BGP is a peering relationship between BGP routers within the same autonomous system. eBGP, or external BGP describes a

peering relationship between BGP routers in different autonomous systems. It is an important distinction to make.

In the diagram below, R1 and R2 are eBGP peers. R2 and R3 and iBGP peers.

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BGP Next-Hop Self

When you have BGP neighbors peering between autonomous systems like R1 and R2 above, BGP uses the the IP address of the router the

update was received from as its ―next hop‖. Routers that receive an update from an eBGP neighbor, it must pass the update to its iBGP

neighbors with-out modifying the next hop attribute.

The next-hop IP address is the IP address of the edge router belonging to the next-hop autonomous system.

For example, let‘s say R1 sends an update to R2 from its 10.1.1.1 serial interface. R2 must use keep the next-hop IP set as 10.1.1.1 when

it passes the update along to R3, its iBGP peer. The problem is that R2 does not know about 10.1.1.1 and so it cannot use it as its next hop

address.

The neighbor [IP address] next-hop-self command solves the problem by advertising itself as the next-hop address. In this example, it

would be applied to R2 so any updates passed along to R3 would use an R2 address as the next-hop.

R2(config)# router bgp 65300

R2(config-router)# neighbor 10.2.2.2 next-hop-self

R2(config)# exit

BGPs Synchronization Rule

The BGP synchronization rule states that a BGP router cannot use or forward new route updates it learns from iBGP peers unless it knows

about the network from another source, like an IGP or static route.

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The idea is to prevent using or forwarding on information that is unreliable and cannot be verified. Remember, BGP prefers reliability and

stability over using the newest, fastest route.

This means that iBGP peers will not update each other unless and IGP is running under the hood. To remove the limitation, use the no

synchronization command under BGP configuration mode. recent versions of IOS have it disabled by default, but it is important topic to

understand.

Resetting BGP Sessions

Internet routers running BGP have enormous routing tables. When a filter is applied, like a route map, changes to BGP attributes occur.

Those changes could affect many of the routes already in the routing table from BGP. Because BGP‘s network list is usually very long,

applying a route map or prefix list after BGP has converged can be disastrous. The router would have to check the filter against every

possible route and attribute combination.

To make matters worse, if it were to apply the filters and pull routes back from neighbors, those changes could then cause another

reconvergence – and on and on. In an effort to avoid that scenario (BGP loves stability), BGP will only apply attribute and network changes

to routes AFTER the filter has been applied. All existing routes stay unchanged.

If the network administrator decides that the filter needs to be applied to all routes, then the BGP instance must be reset – forcing the

entire BGP table to pass through the filter. There are three ways to do this:

Hard reset

Soft reset

Route refresh

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The hard and soft reset options aren‘t discussed here because they are not directly relevant to the exam. You should know though, that

both options are extremely memory-taxing on the router as all the routes must be recomputed. Route refresh was developed to solve the

high memory problems, while still forcing a reset.

The clear ip bgp [ * | neighbor-address] command performs the BGP route refresh.

BGP Configuration

Enabling BGP

Like other routing protocols, BGP must be enabled with the router command. Make sure to include the AS number.

R1(config)# router bgp autonomous-system-number

BGP Peering

Each neighbor must be statically assigned using the neighbor command. If the AS number matches the local router‘s, it is an iBGP

connection. If the AS number is different, it is an eBGP connection.

R1(config-router)# neighbor ip-address remote-as autonomous-system-number

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If a router has a long list of directly connected neighbors, the BGP configuration can start to get long and difficult to follow – especially as

neighbor policies are applied. Peer groups solve that.

Peer groups are groups of peer neighbors that share a common update policy. Updating an entire group of neighbor statements can then

be done with one command. Much easier for large BGP networks. Think of a peer group as a logical grouping of routers that are grouped

under a single name to make changes faster and configurations shorter. Like OUs in Active Directory.

Peer groups not only reduce the number of lines of configuration, but they reduce the ease the overhead of the router. A BGP update

process normally runs for each neighbor. If a peer group is configured, a single update process runs for all routers in the group. Notice

that this means that all of the router inside a peer group must be either all iBGP or eBGP neighbors.

Basic neighbor configuration example:


R1(config-router)# neighbor 10.1.1.1 remote-as 65300



Peer group configuration example:


R1(config-router)# neighbor MINE peer-group

R1(config-router)# neighbor MINE remote-as 65300

R1(config-router)# neighbor 10.1.1.1 peer-group MINE



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BGP Source Address

R1 in the diagram has two different

options when it comes to peering to

R2. It can peer to the physical

interface IP address, 10.1.1.2 or it

can peer to R2′s loopback interface,

192.168.2.2.

If a peer relationship is made using

the physical interface as the source

address, problems can occur if the

interface goes down. In this

scenario, even if R2′s 10.1.1.2

interface drops, it still has

connectivity to R2′s networks via R3

and R2′s other physical interface.

Even though an IGP would still show

R2′s network as accessible, the BGP

peer relationship would drop

because R1 cannot reach its peering

address with R2.

Most implementations recommend

using a loopback address as the BGP

source address for this reason.

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Remember that the loopback address must be added to the IGP running for this to work. This way, if R2′s 10.1.1.2 interface fails, R2 will

still be reachable.

The update-source command accomplishes this. Here’s an example:



R1(config-router)# neighbor 192.168.2.2 update-source loopback0



R2(config-router)# neighbor 192.168.1.1 update-source loopback0

Defining Networks

Network statements in BGP are used differently than in other

routing protocols like EIGRP or OSPF. EIGRP and OSPF use the

network statements to define which interfaces you want to

participate in the routing protocol process.

BGP uses network statements to define which networks the local

router should advertise. Each network doesn‘t have to be

originating from the local router, but the network must exist in the

routing table. The optional mask keyword is often recommended

as BGP supports subnetting and supernetting.

Example:






Understand that by default a BGP router will not advertise a

network learned from one iBGP peer to another. This is why iBGP

is not a good replacement for an IGP like EIGRP and OSPF.

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BGP Path Selection

Unlike most other routing protocols, BGP is not hell-bent on using the fastest path to a given destination. Instead, BGP assigns a long list of

attributes to each path. Each of these attributes can be administratively tuned for extremely granular control of route selections.

BGP also does not load balance across links by default. To select the best route, BGP uses the criteria in the following order:

1. Highest weight

2. Highest local preference

3. Choose routes originated locally

4. Path with the shortest AS path

5. Lowest origin code ( i < e < ? )

6. Lowest MED

7. eBGP route over iBGP route

8. Route with nearest IGP neighbor (lowest IGP metric)

9. Oldest route

10. Neighbor with the lowest router ID

11. Neighbor with the lowest IP address

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Controlling Path Selection

The most common method of controlling the attributes listed above is to use route maps. This allows specific attributes to be changed on

specific routes.

Before we get into route maps, let‘s first discuss the three prominent attributes, weight, local preference, and MED.

Weight

On Cisco routers, weight is the most influential BGP attribute. The weight attribute is proprietary to Cisco and is normally used to select an

exit interface when multiple paths lead to the same destination. Weight is local and is not sent to other routers. It can be a value between

0-65,535. 0 is the default.

In the example below, if you want R2 to prefer to use R1 when sending traffic to 192.168.20.0 then the weight attribute could raised on R2

for R1.




R2(config-router)# neighbor 10.1.1.1 weight 100

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Local Preference

Local preference is not proprietary to Cisco and can be used in a similar fashion to weight. It can be set for the entire router or for a

specific prefix.

Local preferences can range from 0-4,294,967,295, with 100 being the default value. Unlike weight, local preference is propagated to iBGP

neighbors.

Using the diagram above, if an administrator wanted R2 to use R1 when sending traffic to 192.168.20.0, the configuration would look

something like this:


R1(config-router)# bgp default local-preference 500

After the local preference is raised on R1, it will be shared with R2 and R2 will begin using it as its best path to the distant network

(assuming the weight is the same of course).

If you want to set the local preference on specif prefixes, route maps are usually the best option. Below is an example of the local

preference being set using a route map:



R7(config-router)# neighbor 10.10.10.1 route-map lp_example in

R2(config-router)# exit

R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255

R7(config)# route-map lp_example permit 10

R7(config-rmap)# match ip address 7

R7(config-rmap)# set local-preference 300

R7(config-rmap)# exit

R7(config)# route-map lp_example permit 20

R7(config-rmap)# set local-preference 100

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MED

The MED attribute, or multi-exit discriminator is used to influence which path external neighbors use to enter an AS. MED is also much

farther down on the attribute list, so attributes like weight, local preference, AS path length, and origin. The default MED value is 0 and a

lower value is preferred.

A common scenario for MED is when a company has two connections to the same ISP for internet. Weight or local preference could be

used to send outgoing traffic on the higher bandwidth link, but local preference is not shared with routers outside an AS. MED could be set

on one router so ISP routers prefer that path in.

To set the MED on all routes:

R1(config-router)# default-metric value

Here‘s an example using a route map to influence incoming paths to 10.30.30.0/24 using MED:



R7(config-router)# neighbor 10.10.10.1 route-map med_example out


R7(config)# access-list 7 permit 10.30.30.0 0.0.0.255

R7(config)# route-map med_example permit 10

R7(config-rmap)# match ip address 7

R7(config-rmap)# set metric 50

R7(config-rmap)# exit

R7(config)# route-map med_example permit 20

R7(config-rmap)# set metric 150

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Verification

It‘s important that you understand and are able to interpret to results of the show ip bgp command output. It displays the contents of the

local BGP topology database- including the attributes assigned to each network. It is perhaps the most important BGP verification and

troubleshooting tool!

Because BGP uses many attributes and sources routes in a number of ways, the output of the show ip bgp command can be a nit

overwhelming if you don‘t know what you are looking for.

R1# show ip bgp

BGP table version is 21, local router ID is 10.0.22.24

Status codes: s suppressed, d damped, h history, * valid, > best, i – internal

Origin codes: i – IGP, e – EGP, ? – incomplete

Network Next Hop Metric LocPrf Weight Path

*> 10.1.0.0 0.0.0.0 0 32768 ?

* 10.2.0.0 10.0.22.25 10 0 25 ?

*> 0.0.0.0 0 32768 ?

* 10.0.0.0 10.0.22.25 10 0 25 ?

*> 0.0.0.0 0 32768 ?

*> 192.168.0.0/16 10.0.22.25 10 0 25 ?

Attributes

Here‘s a breakdown of some important fields you should consider remembering:

* - An asterisk in the first column means that the route has a valid next hop.

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s (suppressed) – BGP is not advertising the network, usually because it is part of a summarized route.

> - Indicates the best route for a particular destination. These will end up in the routing table.

i (internal) - If the third column has an i in it, it means the network was learned from an iBGP neighbor. If it is blank, it means the network

was learned from an external source.

0.0.0.0 - The fifth column shows the next hop address for each route. A 0.0.0.0 indicates the local router originated the route (examples

include a network command entered locally or a network an IGP redistributed into BGP on the router)

Metric (MED value) – The column titled Metric represents the configured MED values. Recall that 0 is the default and if another value exists,

lower is preferred.

i/?- The last column displays information on how BGP originally learned the route. In the example above, ? is used for each route meaning

they were all redistributed routes into BGP from an IGP. The other option is a question mark, which indicates that network commands were

used to configure the route.

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VPNs & IPSec

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VPN tunnels and IPSec are two topics covered on the exam, but not in great detail. You‘ll need to know enough to verify a sample

configuration and answer straightforward questions on both technologies.

Let‘s start with IPSec.

IPSec Basics

IPSec allows the establishment of a secure connection between two hosts. The IPSec protocol sets up a unidirectional SA (security

association between the two endpoints). Because the association is unidirectional, an SA is created on both ends, resulting in two SAs per

IPSec tunnel.

IPSec tunnels are often used as a backup to a WAN link failure. If a point-to-point WAN circuit drops, an IPSec tunnel can be configured to

automatically be established over the internet to the remote site. When the primary WAN circuit comes back up, the IPSec tunnel is

disconnected.

Floating Static Routes

Configuring an IPSec tunnel to activate when a primary link drops is commonly implemented as a floating static route. The idea is to

configure to IPSec VPN as a static route, but with an administrative distance higher than that of the WAN routing protocol‘s.

If the primary route is active, the backup link is not placed into the routing table because its AD is higher. If the primary route goes down,

the static route becomes active.

To configure a floating static route, make sure you define a higher AD value at the end:

R1(conf)# ip route prefix mask address|interface distance_value

VPN Tunnels

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One major problem with standard IPSec sessions is that they do not support broadcast or multicast traffic. If you want to use an IPSec VPN

in an ―always on‖ fashion, then the tunnel needs to allow routing information to pass through. Of course dynamic routing protocols use

broadcast or multicast to send hellos and updates, so in lies the dilemma.

To get around this issue, a ―tunnel within a tunnel‖ approach can be used. A generic tunnel can be configured within the IPSec tunnel to

allow routing protocol information (along with all the other traffic).

There are generally four ways to do this paired with IPSec:

DMVPN and GET VPN

Both allow the creation of secure, ―on-demand‖, multipoint tunnels.

Virtual Tunnel Interface (VTI)

A secure, ―always-on‖ tunnel that supports multicast traffic. This allows routing protocols to operate within it.

Generic Routing Encapsulation (GRE)

GRE tunnels may be the most common of the bunch – they are also the default tunnel mode on Cisco routers. GRE tunnels support many

layer 3 protocols but perhaps most importantly allow multicast traffic across the tunnel – granting dynamic routing protocol traffic.

Be aware that GRE tunnels add an additional 20 byte IP header as well as a 4 byte GRE tunnel header.

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Branch Office Connectivity

The CCNP ROUTE exam covers several unusual topics related to managing and configuring the connectivity between an HQ site and a

branch office. You need to be familiar with some of the underlying technologies used.

Cisco ISR routers are often a good choice for branch sites as they support a wide variety of incoming services. In smaller offices, a single

ISR may be used for a both remote connectivity and inter-VLAN routing. In that case, know that an Ethernet Switch Module would be

required for the ISR router.

DSL

DSL, or Digital Subscriber Line, can be used as a backup WAN connection to a branch office. DSL uses frequencies not used by TDM phone

systems on a phone line – allowing the extra bandwidth to be used for data connectivity.

Asymmetrical DSL has higher downstream bandwidth than upstream, while with symmetric DSL they are both the same rate.

There are two primary methods for pushing L2 data across a DSL line:

PPoE

Point-to-Point Protocol over Ethernet is the most common method and encapsulates PPP traffic into Ethernet frames.

PPoA

Point-to-Point Protocol over ATM is less common and routes PPP traffic over an ATM network between the customer and the DSL service

provider.

Both options can be configured on a Cisco router to terminate the DSL connectivity. PPoE is especially helpful because it this frees the user

computers from running PPoE

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Cable

Broadband cable providers also provide internet connectivity which can be used for WAN backup or Internet connectivity for telecommuters.

The internet signal is carried on the same line that the television is carried, but a cable modem allows the data traffic to be seperated.

The international standard for sending data over a cable system is Data Over Cable Service Interface Specification (or DOCSIS). Many

different versions of the standard are used throughout the world.

Cable system connections are typically not terminated directly into a Cisco router. Instead, a cable modem demodulates the incoming signal

and converts the traffic to Ethernet frames, which a router can process.

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IPv6

6.

I0I00I0I00II0I0III0I00II0II0I00II0III0I0I0I0I0I00II0I0II0I0II00III0I00II0III0I00

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IPv6 is an important topic – and not just for the exam. The growth of web-based services and diminishing IPv4 addressing will continue to

push organizations towards IPv6, especially on web-facing networks.

Basics

IPv4 addresses are 32 bits long and are represented in dotted-decimal format. IPv6 addresses are 128 bits and are in hexadecimal format.

The first 64 bits of an IPv6 address are reserved for the network portion and the last 64 bits are used for the host portion.

IPv6 Shorthand

The ability to shorten IPv6 addresses is very important to understand because it makes reading and writing them much easier.

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There are two ways to condense an IPv6 address.

1. Leading zeros can be removed in any section.

For example,

0021:0001:0000:030A:0000:0000:0000:0987E

Can be abbreviated as:

21:1:0:30A:0:0:0:987E

2. Sequential sections of all zeros can be shortened to a single double colon.

This can only be used once per address though! Using the same example address above, it can be further shortened to:

21:1:0:30A::987E

Unicast, Multicast, & Anycast

Unicast

Unicast is for sending traffic to a single interface. In IPv6 there are actually two different unicasts types, global unicast andlink-local

unicast.

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Multicast

Unlike IPv4, IPv6 addressing does not support broadcasts. Instead, it has replaced it with multicast (which is a more efficient variation).

This is used for sending traffic to a group of devices.

Anycast

IPv6 supports another, new packet type – anycast. Anycast allows the same address to be used on multiple devices for load balancing and

redundancy. Technically, it is used for sending traffic to the nearest interface in a group. While multiple devices may be running the same

anycast address, only one will be used per packet sent.

Be aware that with IPv6, an interface can be assigned multiple addresses. Here is the list:

- Unicast address

- Link-local address

- loopback (::1/128)

- All nodes multicast (FF00::1)

- Site-local multicast (FF02::2)

- Solicited-nodes multicast

- Default Route (::/0)

Address Assignment

In IPv6, there are three different ways devices are assigned an IP address: manual configuration, using stateless autoconfiguration, or by

using DHCPv6.

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Manual Address Configuration

The first thing to know and manual IPv6 address configuration is that addresses assigned to a router interface use the address/prefix-length

notation instead of the address mask notation. This is so much easier than typing 25.255… after every ip address!

Also, make sure you first enable IPv6 routing with the ipv6 unicast-routing global configuration command.

Use the ipv6 address ipv6-address/prefix-length command to assign an address.

An example of an interface configured with an IPv6 address:

R1# conf t

R1(config)# ipv6 unicast-routing


R1(config-if)# ipv6 address 21:1:0:30A::987E/64

Manual Network Assignment

Another way to manually configure an IPv6 address is to configure the network and allow the host portion to be autopopulated based on

the device‘s MAC address. This can work well because MAC addresses are 64 bits long – the exeact same length as the host portion of an

IPv6 address!

An example with the network portion defined:


R1(config-if)# ipv6 address 21:1:0:30A::/64

Note: Some systems have a 48 bit MAC address. In this case, it flips the 7th bit and inserts 0xFFEE into the middle of the MAC address.

This modified version is called an EUI-64 address. To do this, add the keyword eui-64 to the end of the ipv6 address statement.

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Stateless Autoconfiguration

Stateless autoconfiguration allows a device to self-assign an IP address for use locally without any outside information. Remember that

interfaces using IPv6 will often have more than one IPv6 address assigned, and in this case stateless autoconfiguraiton will generate a link-

local address in addition to any other manually assigned addresses. Link-local addresses are created by starting with the prefix FE80:: and

appending the device‘s MAC address. Since every MAC address should be unique, it works well for auto-generated local IP addresses.

Link-local addresses are not routable within packets and are used for administrative purposes within the local segment. For example, most

IGPs use link-local addresses to for neighbor relationships and the link-local address is listed as the next-hop address in the routing table.

Once a router has created an IPv6 link-local address using stateless autoconfiguration, it used NDP to make sure it is actually unique within

the local network. NDP stands for Neighbor Discovery Protocol. NDP uses ICMP packets as part of the neighbor discovery process.

To configure stateless autoconfiguration, use the ipv6 address autoconfig command.

Example:


R1(config-if)# ipv6 address autoconfig

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IPv6 Routing

Static Routes

The configuration for IPv6 static routes is identical to IPv4, except for the ipv6 route keywords instead of ip route and the addresses will

obviously look different. Other than that, it is exactly the same!

An example of a static IPv6 default route:

R1(config)# ipv6 route ::/0 serial1/1

An example of an IPv6 static route with a next-hop address:

R1(config)# ipv6 route 2003:2:1:A::/64 2003:2:1:F::1

To view the IPv6 routes in the routing table, use the command show ipv6 route.

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IPv6 EIGRP

There are many differences in the way EIGRP is configured and with IPv6. It still sends hellos out every 5 seconds to its neighbors, but

when running EIGRP with IPv6 addresses, it uses the multicast address FF02::A.

EIGRP messages are exchanged using the link-local address as the source address and perhaps the biggest difference is that there is no

network command! Instead, EIGRP routing is enabled on each participating interface.

Also, EIGRP running IPv6 requires a router ID be configured. The format is that of an IPv4 address - 32 digits and it can be a private

address (non-routable) with no issues.

The last major change is that the EIGRP process starts in the shutdown state. You have to issue a no shut to bring it up on the router.

To configure IPv6 EIGRP:


!

R1(config)# ipv6 router eigrp AS

R1(config-rtr)# router-id ipv4-address

R1(config-rtr)# no shut

R1(config-rtr)# exit

!

R1(config)# interface type number

R1(config-if)# ipv6 eigrp AS

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OSPFv3

OSPFv3 is an updated version of OSPF designed to accommodate IPv6 natively. Most of the configuration and function is identical to its

predecessor, but a few changes were made – starting with messaging.

OSPFv3 uses the multicast address FF02::5 and FF02::6, but like EIGRP, it now uses its link-local address as the source address in

advertisements. Also, it‘s possible to run multiple instances of OSPFv3 on each link.

Like the IPv6 implementation of EIGRP, a 32 bit router ID must be manually created. It will not automatically create one based on highest

loopback or interface address. The RID that is assigned will then be used to determine the DR and BDR on a segment (highest wins).

OSPFv3 has dropped its native authentication options. Instead, it relies on the underlying authentications built into IPv6, like IPSec.

Configuration

The configuration is now done on each individual interface. The following is an example configuration:


!

R2(config)# ipv6 router ospf 100

R2(config-rtr)# router-id 10.10.10.1

R2(config-rtr)# area 1 stub no-summary


!

Continued…

R2(config)# interface gig1/1

R2(config-if)# ipv6 address 2003:2:1:2::1/64

R2(config-if)# ipv6 ospf 100 area 0

!

R2(config)# interface gig1/2

R2(config-if)# ipv6 address 2003:2:1:A::1/64

R2(config-if)# ipv6 ospf 100 area 1

R2(config-if)# ipv6 ospf priority 30

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MP-BGP

MP-BGP, or multiple protocol BGP, was outlined in RFC 2858 and includes extensions to the original BGP standard that allows support for

other protocols – one of which is IPv6! The command address-family was added to specify which new protocol functionality is being

configured and is used when applying IPv6 addressing.

Like EIGRP and OSPFv3, an IPv4 address must be configured as a router ID. The BGP configuration is not done at the interface level, it still

is done in router configuration mode. The major difference is that neighbors must be first defined under router BGP configuration mode

and then ―activated‖ under IPv6 address-family mode submode. Networks and other parameters are also configured under IPv6 address-

family mode submode.

Configuration

Like this…


!


R3(config-rtr)# router-id 10.10.10.10

R3(config-rtr)# neighbor 2003:76:1:1::10 remote-as 700

R3(config-rtr)# address-family ipv6 unicast

R3(config-rtr-af)# neighbor 2003:76:1:1::10 activate

R3(config-rtr-af)# network 2003:2:2::/48

R3(config-rtr-af)# exit


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Migrating to IPv6

Three options exist for transitioning from IPv4 to IPv6: dual stack, tunneling, or NAT.

Dual Stack

This involves running IPv4 alongside IPv6 on the same system.

Tunneling

This option allows you to encapsulate IPv6 traffic within an IPv4 header.

NAT

A new network translation extension, NAT-PT allows IPv6-to4 translation.

Dual Stack Continued…

Using a dual-stack transition allows servers, clients, and applications to be slowly moved to IPv6. Both protocols can run cuncurently,

neither communicating with the other. If both IPv4 and IPv6 are running on a server for example, IPv6 will be used.

Configuration Example

R1# config t


R1(config)# ipv6 cef

!

R1(config)# interface serial1/0/1

R1(config-if)# ip address 192.168.1.1

R1(config-if)# ipv6 address 2001:1:3:1::1/64

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IPv6 Tunneling

Dual-stacking IPv4 alongside IPv6 on systems works well, but it requires most of your infrastructure to support IPv6. In many cases, the

network core does not support IPv6 or it has not been implemented. IPv6 tunnels solve this problem by allowing IPv6 islands to exist and

bridging them over IPv4 systems.

Because IPv6 tunnels provide virtual IPv6 connectivity through an IPv4 transport, it does not matter what specific IPv4 transport is used.

The only requirement is that there is end-to-end IPv4 connectivity between both ends.

Manual Tunnels

The tunnels discussed here are from one router to another. The source router (RouterA) encapsulates the IPv6 traffic in IPv4 headers, then

forwards it to the other end of the tunnel (Router B). Router B then decapsulates the packets and forwards them on to their destination

using native IPv6.

Manual IPv6 tunnels are easy to configure using the tunnel mode ipv6ip command. Using the Router A/B example above, the

configuration on Router A would look something like this:

RouterA(config)# interface tunnel0

RouterA(config-if)# ipv6 address 2001:2:0:7::/64

RouterA(config-if)# tunnel source 10.1.1.1

RouterA(config-if)# tunnel destination 10.3.3.1

RouterA(config-if)# tunnel mode ipv6ip

RouterA(config-if)# exit

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GRE Tunnels

First, GRE tunnels are the default tunnel method on Cisco routers. GGRE tunnels are very flexible and work over most protocols.

The configuration is exactly the same as the manual configuration example above, but you do not have to specify the tunnel mode. Also,

routing protocols can be enabled on GRE tunnel interfaces just as if they were physical interfaces.

6to4 Tunnels

6to4 tunnels are similar to the manual tunnel, but set up the tunnel dynamically.

6to4 tunnels use 2002::/16 IPv6 addresses in front of the 32 bit IPv4 address of the edge router – creating a 48 bit prefix. Each router on

both sides of the tunnel needs a route to its peer. They only support static and BGP routes, so be careful.

Configure the tunnel as if it was a manual tunnel, using the IPv4 address as the source, but don‘t enter a destination. The tunnel requires

an IPv6 address using the method just described. Finally, use the command tunnel mode ipv6ip 6to4.

NAT

Translation is a unique solution because it allows IPv4 devices to communicate with IPv6 devices without the dual stack requirement. NAT-

PT allows bidirectional translation services.

1. To enable NAT-PT IPv4 to IPv6 translation on a router, the first step is to use the ipv6 nat command on each interface participating in

the translation.

2. The second step is to define at least on NAT-PT prefix. Only traffic matching the prefix will be translated. To apply it globally on the

router, enter ipv6 nat prefix/prefix_length in global configuration mode. To apply it to traffic on a specific interface, enter ipv6 nat

prefix/prefix_length in interface configuration submode.

3. Define the address mappings (either static or dynamic) using the options discussed below.

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Static NAT-PT

For an IPv6 to IPv4 static mapping:

R1(config)# ipv6 nat v6v4 source ipv6_address ipv4_address


R1(config)# ipv6 nat v4v6 source ipv4_address ipv6_address

Dynamic NAT-PT

There are many ways to implement dynamic NAT using IPv6, but at its most basic level a pool of addresses is created and the router

temporarily assigns them to hosts as they need them.


R1(config)# ipv6 nat v4v6 pool name begining_ipv6 ending_ipv6 prefix-length prefix-length

R1(config)# ipv6 nat v4v6 source list (access-list_number | name) pool name


R1(config)# ipv6 nat v6v4 pool name begining_ipv4 ending_ipv4 prefix-length prefix-length

R1(config)# ipv6 nat v6v4 source list (access-list_number | name) pool name

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ROUTING SIMULATION

SCENARIOS

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EIGRP/OSPF Redistribution Simulation Example:

PROBLEM

In this scenario, routers R2 and R3 need to be configured for

redistribution between their repective EIGRP and OSPF Autonomous

Systems.

Only R2 and R3 can be configured and the traffic path from R1 to the

10.1.1.0 network should use the links with the greatest bandwidth.

When completed, router R1 should be able to ping a host in the

10.1.1.0/24 network.

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SOLUTION

First we need to find out 5 parameters (Bandwidth, Delay, Reliability, Load, MTU) of the s1/1 interface (the interface of R2 connected to R4)

for redistribution :

R2# show interface s1/1

Now write down these 5 parameters, notice that we have to divide the Delay by 10 because its metric unit is tens of microsecond.

For example, if we get:

Bandwidth=1544 Kbit

Delay=20000 us

Reliability=255

Load=1

MTU=1500 bytes

…then we would redistribute as follows:

R2#config terminal


R2(config-router)# redistribute eigrp 200 metric-type 1 subnets


R2(config-router)# router eigrp 200

R2(config-router)# distance eigrp 90 109

R2(config-router)# redistribute ospf 1 metric 1544 2000 255 1 1500

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For R3 we use the show interface fa1/1 to get the same 5 parameters.

R3# show interface fa1/1

For example we get Bandwidth=10000 Kbit, Delay=1000 us, Reliability=255, Load=1, MTU=1500 bytes.

Now let‘s configure it the same way we did R2:

R3#config terminal

R3(config)#router ospf 1

R3(config-router)#redistribute eigrp 200 metric-type 1 subnets

R3(config)#exit

R3(config-router)#router eigrp 200

R3(config-router)#redistribute ospf 1 metric 10000 100 255 1 1500

Verification

Perform a ―show ip route‖ on R1 to see the 10.1.1.0/24 network (the network behind R4) in the routing table.

Next, ping from R1 to the network to validate the connectivity.

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Finally perform a traceroute on R1 to the fa1/1 interface of R1 to make sure the traffic is going form R1-R2-R3-R4. This fulfills the ―highest

bandwidth‖ requirement – using the Fast Ethernet links instead of the Serial connection.

IPv6 OSPF Virtual Link Simulation Example:

PROBLEM

In this scenario, two organizations have merged and need to

connect their core routed networks. Luckily, both have already

implemented IPv6 routing using OSPF, but their area numbers do

not fit together well.

You have been tasked with getting their core OSPF routers up

and running using their current area configuration until a full

redesign can be performed.

Currently, R4‘s loopback address cannot be seen in R1‘s routing

table (and vice-versa).

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SOLUTION

You should know by now that in OSPF, all areas must connect back to the backbone area (area 0). In this case, that isn‘t an option because

the directions specifically ask us not to change the current area assignments.

The solution? A virtual link!

We can configure area 1 as a transit area for area 2 using the area virtual-link command.

R2> enable

R2# configure terminal


R2(config-rtr)# area 1 virtual-link 3.3.3.3

(Notice that we have to use neighbor router-id 3.3.3.3, not R2′s router-id 2.2.2.2)

Now onto R3:

R3> enable

R3# configure terminal

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R3(config-rtr)# area 1 virtual-link 2.2.2.2

That‘s all there is to it!

Verification

To verify that R1 has a route to R4‘s loopback interface, use the show ipv6 route command on R1.

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OSPF Simulation Example:

PROBLEM

Sharky‘s Surf ‗n Turf is a fast-growing corporate seafood establishment and needs your help. A new HQ office was recently

constructed with connectivity provided by router R1. Your task is to configure and verify connectivity between the current HQ head-

end router (R2) and the new location (R1).

The physical cabling between R1 and R2 has been completed, but the configuration of OSPF Area 1 needs to be completed to

include ONLY R1 s1/0 and R2 s1/1. The mask should be configured to allow only the two interfaces to participate in the OSPF area.

Also, Area 1 should be configured in a way so that it does not receive any inter-area or external routes (except default routes). The

interfaces have already been configured with the IP addresses shown in the diagram.

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SOLUTION

1. Let’s start with R1.

There are two primary considerations in this scenario. First, Area 1 must be configured to include only R1 and R2‘s interfaces. The

network that the circuit is using is /30, meaning we need to use a wildcard mask for the OSPF area that reflects that /30 address

range. 0.0.0.3 will do just that!

R1#config terminal



The second major consideration for this scenario is that Area 1 should be configured in a way so that it does not receive any inter-

area or external routes (except default routes).

That‘s code for a totally stubby area. In this case, R1 needs to have the stub command applied.

R1(config-router)# area 1 stub


2. Moving on to R2.

The same network wildcard must be applied to R2: R1#config terminal



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R1(config-router)# area 1 stub no-summary


Notice the area 1 stub no-summary command. R2 is the ABR, so if we want Area 1 to be a totally stubby area then we need to use

the no –summary command here.

That‘s it! Fairly simple, but VERY important practice for the exam.

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Policy-Based Routing Simulation Example:

PROBLEM

You‘ve been tasked with providing a routing policy solution to a new web-startup company. They have heavy outbound HTTP traffic

loads and want to use a dedicated frame relay circuit to carry it.

Configure router PBR in such a way that all HTTP traffic traverses the frame relay link to ISP A if the link is up. All other traffic can

go through either link. Only router PBR can be configured and due to network policies, static routes and default routes are not

allowed.

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SOLUTION

1. First we need to create an access list that defines the web traffic.

PBR(config)# access-list 101 permit tcp any any eq www

A source of ―any‖ is used to capture all EIGRP network sources.

2. Now we create a route map that sets the next-hop for the web traffic.

PBR(config)# route-map PBR permit 10

PBR(config-route-map)# match ip address 101

PBR(config-route-map)# set ip next-hop 10.1.1.1

PBR(config-route-map)# exit

PBR(config)# route-map PBR permit 20

PBR(config-route-map)# exit

Notice that the first statement sets the next-hop address for the HTTP traffic and the following route map line (20) allows any other

traffic through unmodified. If line 20 wasn‘t used, the implicit deny would drop any non-web traffic.

3. Last step is to apply the route map to the internal-facing interface on router PBR.

PBR(config)# int fa0/1

PBR(config-if)# ip policy route-map PBR

PBR(config-if)# exit

PBR(config)# exit

4. Verification

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On a host in the internal EIGRP network, generate HTTP traffic destined for the internet.

Next, use the show route-map command to verify that packets are being matched against the new filter.

PBR# show route-map

You should see something like ―Policy routing matches: 12 packets…‖ in the output if your configuration is correct.

92264738-Ccnp-Route-Exam-Guide-v2-5.pdf

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