R4 rejecting LSA3

[ohassairi]

hi

1-

i am trying to simulate the attached network in GNS3

i found that neither R4 nor R1 have route for the network between R2 and R3

it is like if R4 is rejecting LSA3 for network in area 0. but R4 should not do it since it has no link with R2.

it is a bug or bad  design or what ? ios is 12.3(4)T

2-

actually i am trying to find a scenario where a router receive 2 OSPF routes for the same network : one just O and the second is IO. but i can't find it.

thanks for help

[Rolf Fischer]

Hi,

looks like you don't have a link between R2/R3 and R1/R4, so Area 0 has been split into two partitions. You could fix that with a virtual link through Area 1, but the recommended approach would be an additional link in Area 0 which connects the partitions.

2- OSPF path selection always prefers intra-area routes (O) over inter-area routes (O IA), so you should never see both types of routes for the same prefix in a routing-table. (not sure if this was the question ...)

HTH

Rolf

[ohassairi]

1-in the begining there was a link between R1 and R2 and everything was fine. then i tried to simulate the case where this link fails to check if R1 will learn about the network between R2 and R3 (that comes from area1) or no? and the answer was no !!

2-yes i know that O>IA and i am trying to find the scenario where one router has 2 paths for the same subnet one O and the second IA. normally the diagram i shared could be the case but i was surprised that R1 and R4 can't establish the IA in their route tables

[Rolf Fischer]

It is a requirement that Area 0 has to be continuous. What you're seeing with the Backbone Area partitioned is expected. Like mentionied in my first post, you could configure a virtual link through Area 1 as a backup in the event of link failures if physical redundancy is not possible in Area 0.