Implementing a scalable ospf based solution

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04/13/23Instructional Design-Computer Networking - Bridges Educational Group


Implementing a Scalable OSPF-Based Solution

Lessons Summary: Implementing OSPF Multiarea OSPF IPv4 Implementation Troubleshooting Multiarea OSPF Examining OSPFv3



OSPF OverviewCreates a neighbor relationship by exchanging hello

packets Propagates LSAs rather than routing table updates Link: Router interface State: Description of an interface and its relationship

to neighboring routers Floods LSAs to all OSPF routers in the area, not just

directly connected routers Pieces together all the LSAs generated by the OSPF

routers to create the OSPF link-state database Uses the SPF algorithm to calculate the shortest

path to each destination and places it in the routing table



OSPF Hierarchy Example



Minimizes routing table entries Localizes the impact of a topology change within an area

Neighbor Adjacencies: The Hello Packet



SPF Algorithm



Places each router at the root of a tree and calculates the shortest path to each destination based on the cumulative cost

Cost = Reference Bandwidth / Interface Bandwidth (b/s)

Planning to Deploy OSPFPrior to deploying an OSPF routing solution, the following

should be considered:

• IP addressing plan

• Network topology

• OSPF areas

Once the requirements have been assessed, the implementation plan can be created.

Implementing OSPFThe information necessary to implement OSPF routing includes the following:

• The IP addresses to be configured on individual router interfaces.

• A list of routers on which OSPF is to be enabled, along with the OSPF process number to use and the connected networks that are to run OSPF and that need to be advertised (per individual router).

• The area in which each interface is to be configured.

• Metrics that need to be applied to specific interfaces, or OSPF traffic engineering.

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Instructional Design-Computer Networking - Bridges Educational Group


In the implementation plan, OSPF tasks include the following:

• Enabling the OSPF routing protocol, directly on an interface or by using the correct network command under the OSPF routing process configuration mode.

• Assigning the correct area id to the interface, via the OSPF configuration on the interface or under the OSPF routing process configuration mode.

• Optionally configuring the metric to appropriate interfaces.

Verifying OSPFAfter implementing OSPF, verification should confirm proper deployment on

each router.

Verification tasks include verifying:

• Verifying that the appropriate OSPF neighbor relationships and

adjacencies are established

• Verifying that the OSPF LSDB is populated with the necessary

information.

• Verifying that IP routing table is populated with the necessary

information.

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• Verifying that there is connectivity in the network between routers and

to other devices.

• Verifying that OSPF behaves as expected in a case of a topology

change, by testing link failure and router failure events.

Documenting

After a successful OSPF deployment, the solution and verification process and results should be documented for future reference.

Documentation should include:

• A topology map

• The IP addressing plan

• The area hierarchy

• The networks and interfaces included in OSPF on each router

• The default and any special metrics configured

• The verification results.



Enable OSPF RoutingDefine OSPF as the IP routing protocol.

Router(config)#

router ospf process-id

The process-id is an internally used number that identifies the

OSPF routing process. The process-id does not need to match process IDs on other routers It can be any positive integer in the range from 1 to 65535

Define OSPF networks to advertise to OSPF neighbors.

Router(config-router)#

network ip-address [wildcard-mask] area area-id

The ip-address parameter can be a network, a subnet, or the address of a directly connected interface.

The wildcard-mask is an inverse mask used to determine how to

interpret the address.

• The mask has wildcard bits, where 0 is a match and 1 is “don’t care.”

• For example, 0.0.255.255 indicates a match in the first 2 octets.

• The area-id parameter specifies the OSPF area to be associated with the address.

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The Wildcard MaskRecall that a wildcard mask is the inverse of a subnet mask.

An easy way to calculate the inverse of the subnet mask, is to subtract the subnet mask from 255.255.255.255.

For example, the inverse of subnet mask

255.255.255.252 is 0.0.0.3.



Optional method to enable OSPF explicitly on an interface.

Router(config-if)#

ip ospf process-id area area-id

The process-id parameter can be a network, a subnet, or the

address of a directly connected interface.

• The area-id parameter specifies the OSPF area to be associated

with the address.

• Because this command is configured explicitly for the interface, it takes

precedence over the network area command.

Define the Interface Bandwidth

Defines the interface’s bandwidth (optional).

Router(config-if)#

bandwidth kilobits



The kilobits parameter indicates the intended bandwidth in kbps.

For example, to set the bandwidth to 512,000 bps, use the

bandwidth 512 command.

The configured bandwidth is used by routing protocols in the metric

calculation.

The command does not actually change the speed of the interface.



Configuring Single-Area OSPFRouterX(config)#router ospf process-id

Defines OSPF as the IP routing protocolRouterX(config-router)#network address wildcard-mask area area-id

Assigns networks to a specific OSPF area



Configuring Loopback Interfaces


Router ID:

•Number by which the router is known to OSPF •Default: The highest IP address on an active interface at the moment of OSPF process startup•Can be overridden by a loopback interface: Highest IP address of any active loopback interface •Can be set manually using the router-id command


Configuring Multi-Area OSPF



Alternate Multi-Area OSPF



OSPF Router IDA router is known to OSPF by the OSPF router ID number.• LSDBs use the OSPF router ID to differentiate one router from the next.By default, the router ID is the highest IP address on an active

interface at the moment of OSPF process startup.• However, for stability reason, it is recommended that the router id

command or a loopback interface be configured.



Define the Router IDAssign a specific router ID to the router.Router(config-router)#router-id ip-addressAny unique arbitrary 32-bit value in an IP address format (dotted decimal) can be used.If this command is used on an OSPF process that is already active,

then the new router ID takes effect: After the next router reload. After a manual restarting of the OSPF process using the clear ip

ospf process privileged EXEC command.



Verifying the Router-ID


R2# show ip ospfRouting Process “ospf 50” with ID 10.64.0.2<output omitted>


Verifying OSPF



Verifying the OSPF ConfigurationRouterX# show ip protocols

Verifies that OSPF is configuredRouterX# show ip route

Displays all the routes learned by the router



Verifying the OSPF Configuration (Cont.)RouterX# show ip ospf Displays the OSPF router ID, timers, and

statistics



show ip protocolsVerify routing protocol information on the routerR1# show ip protocolsRouting Protocol is “ospf 1”Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not setRouter ID 10.64.0.1Number of areas in this router is 1. 1 normal 0 stub 0 nssaMaximum path: 4Routing for Networks:10.0.0.0 0.255.255.255 area 0Reference bandwidth unit is 100 mbps<output omitted>



show ip ospf neighbors



Verify that the router recognizes OSPF routes

R1# show ip route ospf10.0.0.0/8 is variably subnetted, 3 subnets, 2 masksO IA 10.2.1.0/24 [110/782] via 10.64.0.2, 00:03:05,

FastEthernet0/0R1#Clearing the OSPF Routing TableTo clear all routes from the IP routing table, use:Router# clear ip route * To clear a specific route from the IP routing table, use:Router# clear ip route A.B.C.D



show ip ospf interfaceVerify OSPF configured interfaces.



OSPF Network Types



Broadcast DR /BDR election required since there could be many

devices.• Establishing adjacencies with all routers in a broadcast

network would easily overload a router due to the overhead of maintaining those

adjacencies.• Instead, OSPF routers form full adjacencies with the DR

and BDR only. Packets to all OSPF routers are forwarded to 224.0.0.5.

Packets to the DR / BDR are forwarded to 224.0.0.6.



Broadcast Challenge: Multiple AdjacenciesA challenge of broadcast network is the number of

adjacencies that would be required.• One adjacency for every pair of routers.• This would increase network traffic and load on each router

to manage each individual adjacency.



Designated Router A designated router (DR) and backup designated router

(BDR) solve these challenges because they:• Reduce routing update traffic• Manage link-state synchronizationThe DR is elected and becomes responsible for maintaining

the topology table for the segment.This DR has two main functions: • To become adjacent to all other routers on the network

segment.• To act as a spokesperson for the network. As spokesperson the DR becomes the focal point for

collecting and sending routing information (LSAs).



Backup Designated Router (BDR)For fault tolerance, a second router is elected as the BDR.• The BDR must also become adjacent to all routers on the

network and must serve as a second focal point for LSAs.• However, the BDR is not responsible for updating the other

routers or sending network LSAs. The BDR keeps a timer on the DR's update activity to ensure

that it is operational. • If the BDR does not detect activity from the DR after the

timer expires, the BDR immediately becomes the DR and a new BDR is

elected.



DR/BDRDRs and BDRs are elected on a per-network basis and therefore each network segment has its own DR and BDR.• For example, a router connected to multiple multiaccess

broadcast networks can be a DR on one segment and a regular (DROTHER) router on another segment.

The election process is accomplished dynamically using the Hello protocol.• However, the election can be manually manipulated the ip

ospf priority number interface configuration command.After a DR and BDR have been selected, any router added to

the broadcast network establishes full adjacencies with the DR and BDR only.



Assigning Router PriorityAssign a specific OSPF priority to the router.Router(config-if)#ip ospf priority numberA router interface can have a priority number between 0 -

255: 0 = DROTHER - Router cannot be a DR 1 = Favorable - Default for all routers 255 = Very favorable - Ensures at least of a tie.The priority must be configured before the election takes

place to figure into the election. To display an interface's priority value and other key

information use theshow ip ospf interface command.



The Election of the DRAll neighbors with a priority > 0 are listed.2. The router with highest priority is elected DR.If there is a tie, the highest router IDs are used.3. If there is no DR, the BDR is promoted as DR.4. The neighbor with the next highest priority is elected BDRManipulating the Election ProcessThe DR / BDR maintain these roles until they fail even when more

routers with higher priorities show up on the network. To influence the election of DR & BDR, do one of the following:• Boot up the DR first, followed by the BDR, and then boot all other routers. OR• Shut down the interface on all routers, followed by a no shutdown on the DR, then the BDR, and then all other routers.



Point-to-Point Both routers become fully adjacent to each another.

Usually a serial interface running either PPP or HDLC.• May also be a point-to-point subinterface running Frame

Relay or ATM. No DR /BDR election required since there are only two

devices. OSPF autodetects this type of network.Packets are sent to 224.0.0.5



OSPF over MPLS Multi-Protocol Label Switching (MPLS) is an Internet

Engineering Task Force (IETF) standard architecture that combines the advantages of Layer 3 routing with the benefits of Layer 2 switching.

A unique feature of MPLS is its capability to perform label stacking, in which multiple labels can be carried in a packet.

The top label, which is the last one in, is always processed first. • Label stacking enables multiple LSPs to be aggregated,

thereby creating tunnels through multiple levels of an MPLS network.



OSPF over Layer 3 MPLS VPNThe customer and provider edge routers are running OSPF. • However the internal provider routers do not.The customer has to agree upon OSPF parameters with the

service provider (SP) to ensure connectivity. • These parameters are often governed by the SP.



OSPF over Layer 2 MPLS VPNThe Layer 2 MPLS VPN backbone and the provider routers are not visible to the customer routers. • A neighbor relationship is established directly between

OSPF enabled routers over the MPLS backbone, and behaves in the same

way as on an Ethernet broadcast network therefore DR and BDR

routers are elected



Nonbroadcast Multiaccess (NBMA)Frame Relay, ATM, and X.25 are examples of NBMA

networks.The default OSPF hello and dead intervals on NBMA

interfaces are 30 seconds and 120 seconds, respectively.Although NBMA networks can support more than two

routers, they have no inherent broadcast capability. • This can create reachability issues. To implement broadcasting or multicasting, the router replicates the packets to be broadcast or multicast and sends them individually on each permanent virtual circuit (PVC) to all destinations.• This process is CPU and bandwidth intensive.



DR Election in an NBMA TopologyBy default, OSPF cannot automatically build adjacencies with

neighbor routers over NBMA interfaces.OSPF considers the NBMA environment to function similarly

to other multiaccess media such as Ethernet. • However, NBMA networks are usually hub-and-spoke (star)

topologies using PVCs or switched virtual circuits (SVCs). • In these cases, the physical topology does not provide the

multiaccess capability on which OSPF relies.The election of the DR becomes an issue in NBMA topologies

because the DR and BDR need to have full Layer 2 connectivity with all routers in the NBMA network.

The DR and BDR also need to have a list of all the other routers so that they can establish adjacencies



OSPF over NBMA TopologyThere are five NBMA topology modes of operation:

• Two official OSPF modes described in RFCs

• Three customized Cisco modes.

RFC 2328-compliant modes are as follows:

• Nonbroadcast (NBMA)

• Point-to-multipoint

Cisco modes are as follows:

• Point-to-multipoint nonbroadcast

• Broadcast

• Point-to-point

OSPF NBMA topology modes are configured using the ip ospf network interface configuration command.

• Some modes require that a neighbor be manually configured using the neighbor router configuration command.



SubinterfacesOSPF can also be run over subinterfaces.• A subinterface is a physical interface that can be split into

multiple logical interfaces.• Each subinterface requires an IP subnet. Subinterfaces can be defined as either a point-to-point or

multipoint interface. • A point-to-point subinterface has similar properties to a

physical pointto-point interface.

Note:The ip ospf network command is not required.Define a SubinterfaceRouter(config)#interface serial number.subinterface-number {multipoint| point-to-point}



Using Point-to-point SubinterfacesCharacteristics: • Same properties as any physical point-to-point physical

interface• DR and BDR not required.• One IP subnet per subinterface pair.• Used when only 2 routers need to form an adjacency on a

pair of interfaces.}



Using Multipoint SubinterfacesThe example has one point-to-point subinterface and one

multipoint subinterface. • The multipoint subinterface supports two other routers in a

single Multipoint Frame Relay subinterfaces default to OSPF nonbroadcast mode, which

requires neighbors to be statically configured and a DR and BDR

election.



Troubleshooting Multiarea OSPFVerifying OSPF OperabilityAn essential requirement of your network operation is that the status of

your routers and routing protocols is monitored to ensure network availability for all users.

The 3 basic forms of network monitoring include router SYSLOG files,

SNMP and MIBs, and using show commands.One useful configuration practice is to configure your routers to use

DNSnames in all OSPF show command displays. This feature makes it easier to identify a router because the

router is displayed by name rather than by its router ID or neighbor ID.ip ospf name-lookup is used to configure OSPF to display IP

addresses by their DNS names instead.



One very useful troubleshooting tool is keeping track of events that affect a router's

operation with logging.SYSLOG on a router with a SYSLOG server is the recommended practice,

since without aserver, you are limited to the memory on the router for record keeping.The storage of the logs on a SYSLOG server is useful for trend analysis,

forensic gathering,and troubleshooting viewpoint.This can also help you find system error messages, outages, and a variety of

other networkevents that may have already passed and been lost in a router's memory.You first need to decide the level of logging that you wish to use, with 8

possible options tochoose from.

The logging levels are as follows:



show logging is used to display the addresses and levels associated with the current logging setup as well as any other logging statistics.

You also want to ensure that the SYSLOG entries are stamped with the correct time and date.

service timestamp log datetime localtime show-timezone is used to configure the router to automatically data and time stamp all SYSLOG entries on the router and when sent to the SYSLOG server.

show ip ospf border-routers displays the internal OSPF routing table to an ABR or ASBR.



show ip ospf database displays the contents of the topology database maintained by the router.There is actually a list of various forms of this command that can be used to deliver information about different OSPF link-state advertisements.

show ip ospf database asbr-summary displays a large variety of information in the OSPF database for an ASBR.

show ip ospf database database-summary is used to provide a summary of every type of LSA that has been sent, deleted, or expired

because of Maxage.show ip ospf database external provides information

regarding the OSPF database external LSAs.

show ip ospf database network provides information regarding the OSPF database network LSAs, which includes where the routes came from in the network and which routers are part of the network by OSPF area.

show ip ospf database router provides information regarding the OSPF database router LSAs



show ip ospf neighbor [interface-name | detail] is used to display OSPF neighbor information on a per-interface basis.



show ip ospf neighbor ip-address is used to provide detailed information regarding a specific OSPF network as specified by the IP address.



clear ip ospf process [process-id] is used to completely reset either a specific OSPF process, or all OSPF processes on a router.



The Debugging OSPF Command:Among information provided by this command is the :i. Debug ip ospf packet: this command displays hello packets being sent and

received on your routerii. Debug ip ospf hello: this command displays hello packets being sent and

received on your router. It also displays more information than the debug ospf packet

iii. The debug ip ospf adj: shows DR and DBR elections on a broadcast and non-broadcast multi-access (NBMA) network.

debug ip ospf hello displays information about how OSPF Hellos are operating within the OSPF domain.

debug ip ospf packet displays a LOT of detailed information about each OSPF packet received.Be aware that this produces a set of information for EACH packet

received.



Examining OSPFv3OSPFv3 / OSPFv2 SimilaritiesBasic packet typesHello, DBD, LSR, LSU, LSA• Mechanisms for neighbor discovery andadjacency formation• Interface typesP2P, P2MP, Broadcast, NBMA, Virtual• LSA flooding and aging• Nearly identical LSA types

V2, V3 DifferencesOSPFv3 Is Running per Link Instead of per IP Subnet• A link by definition is a medium over which two nodes can communicate at link

layer• In IPv6 multiple IP subnet can be assigned to a link and two nodes in different

subnet can communicate at link layer therefore OSPFv3 is running per link instead of per IP subnet

• An Interface connect to a link and multiple interface can be connected to a link



Support of Multiple Instance per LinkNew field (instance) in OSPF packet header allow running multiple instance per link• Instance ID should match before packet being accepted• Useful for traffic separation, multiple areas per link and AFAddress Semantic Change in LSARouter and Network LSA carry only topology information• Router LSA can be split across multiple LSAs; Link State ID in LSA header is a

fragment ID• Intra area prefix are carried in a new LSA payload called intra-area-prefix-LSAs• Prefix are carried in payload of inter-area and external LSAGeneralization of Flooding Scope• In OSPFv3 there are three flooding scope for LSAs (link-local scope, area scope,

AS scope) and they are coded in LS type explicitly• In OSPFv2 initially only area and AS wide flooding was defined; later opaque LSAs

introduced link local scope as wellExplicit Handling of Unknown LSAThe handling of unknown LSA is coded via U-bit in LS type• When U bit is set, the LSA is flooded with the corresponding flooding scope, as if

it wasunderstood• When U bit is clear, the LSA is flooded with link local scope• In v2 unknown LSA were discarded



Authentication Is Removed from OSPFAuthentication in OSPFv3 has been removed and OSPFv3 relies now on IPv6

authentication header since OSPFv3 run over IPv6• Autype and Authentication field in the OSPF packet header therefore have

been suppressed.

OSPF Packet format has been changed• The mask field has been removed from Hello packet• IPv6 prefix are only present in payload of Link State update packet

Configuring OSPFv3 in Cisco IOS® Software

Similar to OSPFv2Prefixing existing Interface and Exec mode commands with “ipv6”• Interfaces configured directly Replaces network command• “Native” IPv6 router mode Not a sub-mode of router ospf



Configuration modes in OSPFv3

• Entering router mode

[no] ipv6 router ospf <process ID>• Entering interface mode[no] ipv6 ospf <process ID> area <area ID>• Exec modeshow ipv6 ospf [<process ID>]clear ipv6 ospf [<process ID>]Cisco IOS OSPFv3 Specific AttributesConfiguring area range[no] area <area ID> range <prefix>/<prefix length>• Showing new LSAshow ipv6 ospf [<process ID>] database linkshow ipv6 ospf [<process ID>] database prefix• Configuring authenticationUnder ipv6 router ospf: area 0 authentication ipsec spi 256 md5 ciscoUnder interface:ipv6 ospf authentication ipsec spi 256 md5 cisco

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OSPFv3 Debug CommandsAdjacency is not appearing[no] debug ipv6 ospf adj[no] debug ipv6 ospf hello• SPF is running constantly[no] debug ipv6 ospf spf[no] debug ipv6 ospf flooding[no] debug ipv6 ospf events[no] debug ipv6 ospf lsa-generation[no] debug ipv6 ospf database-timer• General purpose[no] debug ipv6 ospf packets[no] debug ipv6 ospf retransmission[no] debug ipv6 ospf tree



OSPFv3 Configuration Example



Enhanced Routing Protocol Support Cisco IOS OSPFv3Router2#sh ipv6 ospf int pos 3/0POS3/0 is up, line protocol is upLink Local Address FE80::290:86FF:FE5D:A000, Interface ID 7Area 1, Process ID 100, Instance ID 0, Router ID 10.1.1.4Network Type POINT_TO_POINT, Cost: 1Transmit Delay is 1 sec, State POINT_TO_POINT,Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5Hello due in 00:00:02Index 1/1/1, flood queue length 0Next 0x0(0)/0x0(0)/0x0(0)Last flood scan length is 3, maximum is 3Last flood scan time is 0 msec, maximum is 0 msecNeighbor Count is 1, Adjacent neighbor count is 1Adjacent with neighbor 10.1.1.3Suppress hello for 0 neighbor(s)



Enhanced Routing Protocol Support Cisco IOS OSPFv3Router2#sh ipv6 ospf neighbor detailNeighbor 10.1.1.3In the area 1 via interface POS3/0Neighbor: interface-id 8, link-local address

FE80::2D0:FFFF:FE60:DFFFNeighbor priority is 1, State is FULL, 12 state changesOptions is 0x630C34B9Dead timer due in 00:00:33Neighbor is up for 00:49:32Index 1/1/1, retransmission queue length 0, number of

retransmission 1First 0x0(0)/0x0(0)/0x0(0) Next 0x0(0)/0x0(0)/0x0(0)Last retransmission scan length is 2, maximum is 2Last retransmission scan time is 0 msec, maximum is 0 msec



Router2#sh ipv6 routeIPv6 Routing Table - 5 entriesCodes: C - Connected, L - Local, S - Static, R - RIP, B - BGPU - Per-user Static routeI1 - ISIS L1, I2 - ISIS L2, IA - ISIS interareaO - OSPF intra, OI - OSPF inter, OE1 - OSPF ext 1, OE2 - OSPF ext 2OI 2001:db8:FFFF:1::/64 [110/2]via FE80::2D0:FFFF:FE60:DFFF, POS3/0C 2001:db8:1:1::/64 [0/0]via ::, POS3/0L 2001:db8:1:1::1/128 [0/0]via ::, POS3/0L FE80::/10 [0/0]via ::, Null0L FF00::/8 [0/0]via ::, Null0



Cisco IOS OSPFv3 Database Display



OSPFv3 on IPv6 Tunnels over IPv4



Lessons Learned:What is OSPF and its configurationTroubleshooting OSPF OSFV3 what it is and its configuration.



Implementing a scalable ospf based solution

Technology

ospf routers

ospf configuration

ospf networks

ospf tasks

ospf lsdb

ospf overview

ospf neighbors

ospf routing solution