1 © 2003, Cisco Systems, Inc. All rights reserved. CCNP 1 v3.0 Module 8 Route Optimization.

1© 2003, Cisco Systems, Inc. All rights reserved.

CCNP 1 v3.0 Module 8Route Optimization


Route Optimization

You can control when a router exchanges routing updates and what those updates contain.

1. routing update control

2. policy-based routing

3. route redistribution


A route optimization example

RTA(config)#router rip

RTA(config-router)#network 10.0.0.0

Send and Receive RIP Updates





Include 10.0.0.0 subnets in updates

10.2.2.0 /2410.3.3.0 /2410.4.4.0 /24

10.1.1.0 /2410.2.2.0 /2410.4.4.0 /24

10.1.1.0 /2410.3.3.0 /2410.4.4.0 /24





Send and Receive RIP Updates


Passive Interfaces



RTA(config-router)#passive-interface e0

Passive Interfaces receive, but don’t send updates


Passive Interfaces and DDR

• You can use the passive-interface command on WAN interfaces to prevent routers from sending updates to link partners.

• There may be several reasons to squelch updates on the WAN.

– If connected by a dial-on-demand ISDN link regular RIP updates will keep the link up constantly, and increase the bill from the provider.


Passive Interfaces and DDR



RTA(config-router)#passive-interface bri0

RTA(config-router)#redistribute static

RTA(config-router)#exit

RTA(config)#ip route 172.16.1.0 255.255.255.0 bri0

...

RTX(config)#ip route 0.0.0.0 0.0.0.0 Bri0


Passive Interfaces

The ‘passive-interface’ command works differently with the different IP routing protocols that support it.

– OSPF: the network address of the passive interface appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received via a passive interface.

– EIGRP: the router stops sending hello packets on passive interfaces. When this happens, the EIGRP router can’t form neighbor adjacencies on the interface or send and receive routing updates.


Route Filters

• Configuring an interface as passive prevents it from sending updates entirely, but there are times when you need to suppress only certain routes in the update from being sent or received.

1. Use the ‘distribute-list’ command

2. Use route-maps


Route Filters

By referencing an access-list, the ‘distribute-list’ creates a route filter -- a set of rules that precisely controls what routes a router will send or receive in a routing update.


Route Filters

Route filters may be needed to enforce a routing policy that’s based on some external factor such as:– link expense

– administrative jurisdiction

– security concerns

– overhead reduction—prevents access routers from receiving the complete (and possibly immense) core routing table


Route Filters


Route Filters

When applied to inbound updates, the syntax for configuring a route filter is as follows:

Router(config-router)#distribute-list access-list-number in [interface-name]


Route Filters

When applied to outbound updates, the syntax can be more complicated:

Router(config-router)#distribute-list access-list-number out [interface-name | routing-process | as-number]


Outbound Route Filters (global)



RTA(config-router)#distribute-list 24 out


RTA(config)#access-list 24 deny 10.1.1.0 0.0.0.255

RTA(config)#access-list 24 permit any

By not specifying an interfacethe distribute list is appliedto all interfaces that wouldsend a routing update.


Outbound Route Filters (interface)



RTA(config-router)#distribute-list 24 out interface s2


RTA(config)#access-list 24 deny 10.1.1.0 0.0.0.255

RTA(config)#access-list 24 permit any

This time, since an interface wasspecified, the distribute list only applies to routing updates thatwould exit that interface.


Inbound Route Filters (global)

RTZ(config)#router rip

RTZ(config-router)#network 10.0.0.0

RTZ(config-router)#distribute-list 16 in

RTZ(config-router)#exit

RTZ(config)#access-list 16 deny 10.1.1.0 0.0.0.255

RTZ(config)#access-list 16 permit any

This example is from RTZ’sperspective.

That is why the distribute listis being applied inbound.


Inbound Route Filters (interface)

S0


RTZ(config-router)#network 10.0.0.0

RTZ(config-router)#distribute-list 16 in interface s0


RTZ(config)#access-list 16 deny 10.1.1.0 0.0.0.255

RTZ(config)#access-list 16 permit any

This example is also from RTZ’sperspective.

That is why the distribute listis being applied inbound.


Route Filters

For each routing protocol, you can have an inbound and outbound global and interface route filter:


RTZ(config-router)#distribute-list 1 in

RTZ(config-router)#distribute-list 2 out

RTZ(config-router)#distribute-list 3 in e0

RTZ(config-router)#distribute-list 4 out e0


Route Filters

Use show ip protocols to display route filters:

RTZ#show ip protocols

Routing Protocol is "rip"

Sending updates every 30 seconds, next due in 25 seconds

Invalid after 180 seconds, hold down 180, flushed after 240

Outgoing update filter list for all interfaces is 2

Ethernet0 filtered by 4

Incoming update filter list for all interfaces is 1

Ethernet0 filtered by 3


“Passive” EIGRP interfaces

• A passive interface can’t send EIGRP hellos, which thus prevents adjacency relationships with link partners.

• An administrator can create a “psuedo” passive EIGRP interface by using a route filter that suppresses all routes from the EIGRP routing update.


“Passive” EIGRP interfaces

RTA(config)#router eigrp 364


RTA(config-router)#distribute-list 5 out interface s0


RTA(config)#access-list 5 deny any

Denies all routing updates


Administrative Distance

• A routing protocol’s administrative distance rates its trustworthiness as a source of routing information. – Administrative distance is an integer from 0 to 255. The

lowest administrative distance has the highest trust rating.

– An administrative distance of 255 means the routing information source cannot be trusted at all and should be ignored.

– An administrative distance of zero is reserved for connected interfaces, and will always be preferred.



• Specifying administrative distance values enables the Cisco IOS software to discriminate between sources of routing information.

• The software always picks the route whose routing protocol has the lowest administrative distance.

• Although we can’t easily compare apples with oranges, we can, for example, instruct the router to always choose oranges over apples.





• When using multiple IP routing protocols on a router, the default distances almost always suffice.

• However, some circumstances call for changing the administrative distance values on a router.


Changing Administrative Distance

• If, for example, a router is running both IGRP and OSPF, it may receive routes to the same network from both protocols. The default administrative distances favor IGRP routes over OSPF routes:

I 10.0.0.0 [100/10576] via 192.168.0.1, Serial0

0 10.0.0.0 [110/192] via 172.17.0.1, Serial1



• But since IGRP doesn’t support CIDR, you may want the router to use the OSPF route instead. In this case, you can configure the local router to apply a custom administrative distance to all OSPF routes:

RTZ(config)#router ospf 1

RTZ(config-router)#distance 95



• With the ‘distance 95’ command, RTZ compares the IGRP and OSPF routes and comes up with a different result:

I 10.0.0.0 [100/10576] via 192.168.0.1, Serial0

0 10.0.0.0 [ 95/192] via 172.17.0.1, Serial1



• You can also apply the ‘distance’ command with optional arguments to make changes to selected routes based on where they originate.

• The expanded syntax of the ‘distance’ command is as follows:

Router(config-router)#distance weight [source-ip-address source-mask (access-list-number | name)]



• Using the optional arguments, we can configure a router to apply an administrative distance of 105 to all RIP routes received from 10.4.0.2.


RTZ(config-router)#distance 105 10.4.0.2 255.255.255.0



• Or, we can configure a router to apply an administrative distance of 97 to specific RIP routes received from 10.3.0.1.


RTZ(config-router)#distance 97 10.3.0.1 255.255.255.0 2


RTZ(config)#access-list 2 permit 192.168.3.0 0.0.0.255



The result from both previous examples:

RTZ#show ip route

R 192.168.5.0/24 [105/1] via 10.4.0.2, 00:00:02, Serial1

10.0.0.0/16 is subnetted, 5 subnets

R 10.2.0.0 [120/1] via 10.3.0.1, 00:00:02, Serial0

C 10.3.0.0 is directly connected, Serial0

R 10.1.0.0 [120/2] via 10.3.0.1, 00:00:02, Serial0

C 10.4.0.0 is directly connected, Serial1

R 192.168.1.0/24 [120/3] via 10.3.0.1, 00:00:02, Serial0

R 192.168.2.0/24 [120/2] via 10.3.0.1, 00:00:02, Serial0

R 192.168.3.0/24 [97/1] via 10.3.0.1, 00:00:02, Serial0


Policy Routing

• You can use the ‘ip route’ command to dictate which path a router will select to a given destination.

• However, through policy routing, you can manually program a router to choose a route based not only on destination, but on source as well.


Policy Routing

• Human factors such as monetary expense, organizational jurisdiction, or security issues can lead administrators to establish policies, or rules that routed traffic should follow.

• Left to their default behavior, routing protocols may arrive at path decisions that conflict with these policies.


Policy Routing

• Administrators use policy routing to override dynamic routing and take precise control of how their routers handle certain traffic.

• Although policy routing can be used to control traffic within an AS, it is typically used to control routing between autonomous systems (ASs).

• Policy routing is used extensively with exterior gateway protocols (EGPs), such as BGP.


Policy Routing – Route Maps

The ‘route-map’ command is used to configure policy routing, which is often a complicated task.

A route map is defined using the following syntax:

Router(config)# route-map map-tag [permit | deny] [sequence-number]

Router(config-map-route)#


Policy Routing – Route Maps

You can use the optional sequence-number to indicate the position a new route map is to have in the list of route maps already configured with the same name.

If you do not specify a sequence number, the first route map condition will be automatically numbered as 10.


Policy Routing

Once you have entered the route-map command, you can enter ‘set’ and ‘match’ commands in the route-map configuration mode. – Each route-map command has a list of match and set

commands associated with it.

– The match commands specify the match criteria—the conditions that should be tested to determine whether or not to take action.

– The set commands specify the set actions—the actions to perform if the match criteria are met.


Policy Routing Example



Assume for this example that the policy we want to enforce is this:

Internet-bound traffic from 192.168.1.0 /24 is to be routed to ISP1, and Internet-bound traffic from 172.16.1.0 /24 is to be routed to ISP2.





RTA(config)#access-list 1 permit 192.168.1.0 0.0.0.255


RTA(config)#route-map ISP1 permit 10

RTA(config-route-map)#match ip address 1

RTA(config-route-map)#set interface s0



RTA(config-route-map)#set interface s1



• We have actually configured two policies with these commands:– The ISP1 route map will match access-list 1, and route

traffic out S0 toward ISP1.

– The ISP2 route map will match access-list 2, and route that traffic out S1 toward ISP2.

• The final step is to apply each route map to the appropriate interface on RTA using the ‘ip policy route-map’ command.



RTA(config)#interface e0

RTA(config-if)#ip policy route-map ISP1

RTA(config-if)#interface e1

RTA(config-if)#ip policy route-map ISP2

With the route maps applied to the appropriate LAN interfaces, we have successfully implemented policy routing.



Assume for this example that our policy allows for traffic from from 192.168.1.0 /24 to be routed to ISP2, if the link to ISP1 is down.

And, Internet-bound traffic from 172.16.1.0 /24 can be routed via ISP1, if the link to ISP2 is down.







RTA(config-route-map)#set interface s0 s1



RTA(config-route-map)#set interface s1 s0

Second interface will be used if first is unavailable


Route Redistribution

• Cisco routers support up to 30 dynamic routing processes.

– A router can run RIP, OSPF, IGRP, IS-IS, EIGRP, IPX RIP, RTMP (AppleTalk), and other protocols simultaneously.

– Most of these routing protocols allow an administrator to configure multiple processes of the same routing algorithm; RIP is a notable exception.


Multiple Routing Protocols

RTA#show running-config

router ospf 24

network 10.2.0.0 0.0.255.255 area 0

!

router ospf 46

network 192.168.2.0 0.0.0.255 area 2

!

router igrp 53

network 172.16.0.0

network 172.17.0.0

!

router igrp 141

network 10.0.0.0

network 192.168.3.0



• To support multiple routing protocols within the same internetwork efficiently, routing information must be shared among the different routing protocols.

For example, routes learned from a RIP process may need to be imported into an IGRP process.



• The process of exchanging routing information between routing protocols is called route redistribution.



• Route redistribution can be one-way (that is, one protocol receives the routes from another) or two-way (that is, both protocols receive routes from each other).

• Routers that perform redistribution are called boundary routers because they border two or more ASs or routing domains.



• Why configure redistribution?

1. You want to run IGRP/EIGRP in one or more areas in a mixed vendor environment

2. You want to support legacy UNIX systems that support RIP only, but use a more scalable protocol elsewhere.

3. You need a temporary fix during a prolonged upgrade from older protocols and hardware to newer, more scalable solutions.





• Because each routing process places substantial demands on the router’s memory and CPU resources, only boundary routers should run more than one routing process for the same routed protocol, and only when absolutely necessary.


Configuring Redistribution

• In the following example:

RTB will inject routes learned from the RIP domain into the EIGRP domain.

RIP routers will not learn about routes from the EIGRP domain. (one-way route distribution)

RIP routers can use a default route to handle any traffic bound for non-local destinations.



RTB(config)#router rip

RTB(config-router)#network 172.16.0.0

RTB(config-router)#router eigrp 24


RTB(config-router)#redistribute rip metric 10000 1 255 1 1500



The metric argument sets up the values used by EIGRP to translate the metric from RIP’s hop count, to EIGRP’s composite metric.

When used with IGRP/EIGRP, the metric keyword sets the bandwidth value (in kbps), the delay (in tens of microseconds), the reliability (out of 255), the load (out of 255), and finally, the maximum transmission unit (MTU).



• These five values constitute the seed metric in our example.

• The seed metric is the initial metric value of an imported route.



• Once imported into the EIGRP AS, a RIP route will begin its life as an EIGRP route with a composite metric derived from these values—regardless of its former RIP metric. – So, using the configuration the previous example, RIP

routes with metrics of 2, 6, and 14 will all be redistributed with the same EIGRP metric value.

– However, as the imported route is propagated to other EIGRP routers, its metric values will increment according the rules of EIGRP.





RTA’s Routing Table (after redistribution)

C 172.24.0.0/16 is directly connected, Serial0




D 172.28.0.0/16 [90/2681856] via 172.20.0.2, 00:00:02, Serial1

D 172.29.0.0/16 [90/2681856] via 172.21.0.2, 00:00:02, Serial3

D EX 172.17.0.0/16 [170/2195456] via 172.24.0.1, 00:00:02, Serial0

D EX 172.18.0.0/16 [170/2195456] via 172.24.0.1, 00:00:02, Serial0

D EX 172.19.0.0/16 [170/2195456] via 172.24.0.1, 00:00:02, Serial0

D EX 172.20.0.0/16 [170/2195456] via 172.24.0.1, 00:00:02, Serial0

D EX 172.21.0.0/16 [170/2195456] via 172.24.0.1, 00:00:02, Serial0



RTB’s Routing Table



R 172.17.0.0/16 [120/1] via 172.16.0.2, 00:00:02, Serial0

R 172.18.0.0/16 [120/1] via 172.16.0.2, 00:00:02, Serial0

R 172.19.0.0/16 [120/1] via 172.16.0.2, 00:00:02, Serial0

R 172.20.0.0/16 [120/2] via 172.16.0.2, 00:00:02, Serial0

R 172.21.0.0/16 [120/2] via 172.16.0.2, 00:00:02, Serial0

D 172.25.0.0/16 [90/2681856] via 172.24.0.2, 00:00:02, Serial1

D 172.26.0.0/16 [90/2681856] via 172.24.0.2, 00:00:02, Serial1

D 172.37.0.0/16 [90/2681856] via 172.24.0.2, 00:00:02, Serial1

D 172.28.0.0/16 [90/3193856] via 172.24.0.2, 00:00:02, Serial1

D 172.29.0.0/16 [90/3193856] via 172.24.0.2, 00:00:02, Serial1





RTC’s Routing Table (after redistribution)





R 172.20.0.0/16 [120/1] via 172.17.0.2, 00:00:02, Serial1

R 172.21.0.0/16 [120/1] via 172.19.0.2, 00:00:02, Serial3

Since the table is not complete, we may need to add a default route:

RTC(config)#ip route 0.0.0.0 0.0.0.0 172.16.0.1



Two-way redistribution





RTB(config-router)#redistribute rip metric 10000 1 255 1 1500

RTB(config-router)#router rip


RTB(config-router)#redistribute eigrp 24 metric 2



• Notice that the syntax of the metric keyword varies depending on the routing protocol it is used with.

• For RIP, OSPF, and the metric option is followed by a single number that represents the metric value (hop count, cost, etc.).

• For IGRP and EIGRP, the metric option is followed by five values that represent bandwidth, delay, reliability, load and MTU.


OSPF WARNING

• Whenever there is a major net that is subnetted, you need to use the keyword ‘subnets’ to redistribute protocols into OSPF.

• Without this keyword, OSPF only redistributes major nets that aren’t subnetted.

• For example, to inject EIGRP routes, including subnets, into an OSPF area, use the command:

redistribute eigrp 24 metric 100 subnets


A complex example



RTB(config-router)#redistribute rip

RTB(config-router)#redistribute connected

RTB(config-router)#redistribute static

RTB(config-router)#default-metric 10000 100 255 1 1500

RTB(config-router)#router rip


RTB(config-router)#redistribute eigrp 24

RTB(config-router)#redistribute connected

RTB(config-router)#redistribute static

RTB(config-router)#default-metric 2


Default-metric

• We can simplify our redistribution configuration by using the ‘default-metric’ command instead of including the same seed metric with each redistribute statement.

• Whenever the redistribute command is used and the metric is not specified, the router will use the default metric value as the seed metric.


Redistribution Example

• Phase 1: configuring a RIP network

• Phase 2: adding OSPF to the core of a RIP network

• Phase 3: adding OSPF areas


Phase 1: Configuring a RIP Network


Phase 2: Adding OSPF to the Core of a RIP Network


Phase 3: Adding OSPF Areas


Summary

• Key IOS Route Optimization Features

Routing update control

Policy-based routing

Route redistribution

1 © 2003, Cisco Systems, Inc. All rights reserved. CCNP 1 v3.0 Module 8 Route Optimization.

Documents

router rip rtaconfigrouter

passive interfaces rtaconfig

cisco systems

static rtaconfigrouter

route optimization slide

eigrp router

route filters route

route redistribution