OSPF (Open Shortest Path First)

Note:

All routers will generate Router (Type 1) LSAs. For example, Router A

will generate a Type 1 LSA that contains the status of links FastEthernet 0/0

and FastEthernet 0/1. This LSA will be flooded to all other routers in Area 1.

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Designated Routers will generate Network (Type 2) LSAs. For example, if

Router C was elected the DR for the multi-access network in Area 1, it

would generate a Type 2 LSA containing a list of all routers attached to it.

Area Border Routers (ABRs) will generate Network Summary (Type 3)

LSAs. For example, Router C is an ABR between Area 0 and Area 1. It will

thus send Type 3 LSAs into both areas. Type 3 LSAs sent into Area 0 will

contain a list of networks within Area 1, including costs to reach those

networks. Type 3 LSAs sent into Area 1 will contain a list of networks

within Area 0, and all other areas connected to Area 0. This allows Area 1 to

reach any other area, and all other areas to reach Area 1.

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The OSPF Metric

OSPF determines the best (or shortest) path to a destination network using a

cost metric, which is based on the bandwidth of interfaces. The total cost of

a route is the sum of all outgoing interface costs. Lowest cost is preferred.

Cisco applies default costs to specific interface types:

Type Cost

Serial (56K)                        1785

Serial (64K)                       1562

T1 (1.544Mbps)                64

Token Ring (4Mbps)        25

Ethernet (10 Mbps)         10

Token Ring (16 Mbps)     6

Fast Ethernet                     1

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The first command, router ospf 1, enables the OSPF process. The “1”

indicates the OSPF process ID, and can be unique on each router. The

process ID allows multiple OSPF processes to run on the same router. The

router-id command assigns a unique OSPF ID of 1.1.1.1 for this router.

Note the use of a wildcard mask instead of a subnet mask in the network

statement. With OSPF, we’re not telling the router what networks to

advertise; we’re telling the router to place certain interfaces into specific

areas, so those routers can form neighbor relationships. The wildcard mask

0.0.255.255 tells us that the last two octets can match any number.

The first network statement places interface E0 on Router A into Area 1.

Likewise, the second network statement places interface S0 on Router A into

Area 0. The network statement could have been written more specifically:

RouterA(config)# router ospf 1

RouterA(config-router)# network 172.16.1.2 0.0.0.0 area 1

RouterA(config-router)# network 172.17.1.1 0.0.0.0 area 0

In order for Router B to form a neighbor relationship with Router A, its

connecting interface must be put in the same Area as Router A:

RouterB(config)# router ospf 1

RouterA(config-router)# router-id 2.2.2.2

RouterB(config-router)# network 172.17.1.2 0.0.0.0 area 0

RouterB(config-router)# network 172.18.1.1 0.0.0.0 area 2

If Router B’s S0 interface was placed in a different area than Router A’s S0

interface, the two routers would never form a neighbor relationship, and

never share routing updates.

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However, please note that the passive-interface command works differently

with OSPF than with RIP or IGRP. OSPF will no longer form neighbor

relationships out of a “passive” interface, thus this command prevents

updates from being sent or received out of this interface:

RouterC(config)# router ospf 1

RouterC(config-router)# network 10.4.0.0 0.0.255.255 area 0

RouterC(config-router)# network 10.2.0.0 0.0.255.255 area 0

RouterC(config-router)# passive-interface s0

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Router C will not form a neighbor adjacency with Router B.

It is possible to configure all interfaces to be passive using the passiveinterface

default command, and then individually use the no passiveinterface

command on the interfaces that neighbors should be formed on:

RouterC(config)# router ospf 1

RouterC(config-router)# network 10.4.0.0 0.0.255.255 area 0

RouterC(config-router)# network 10.2.0.0 0.0.255.255 area 0

RouterC(config-router)# passive-interface default

RouterC(config-router)# no passive-interface e0

Always remember, that the passive-interface command will prevent OSPF

(and EIGRP) from forming neighbor relationships out of that interface. No

routing updates are passed in either direction.