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OSPF Query

I am sorry, this is not the right OSPF topology. My test OSPF setup is shown in the attachment and I am particularly interested in the routing table on RTR-3 and RTR-4.
In a normal situation the routing table on both the routers are fine. However when I shutdown the interface Fe0/1 on RTR-3, there are no routes(on RTR-4) for the the network 172.16.0.0/16.
I can see all the OSPF neighbours on RTR-4. What is the reason?

2 ACCEPTED SOLUTIONS

Accepted Solutions
Cisco Employee

Re: OSPF Query

avilt wrote:

I am sorry, this is not the right OSPF topology. My test OSPF setup is shown in the attachment and I am particularly interested in the routing table on RTR-3 and RTR-4.
In a normal situation the routing table on both the routers are fine. However when I shutdown the interface Fe0/1 on RTR-3, there are no routes(on RTR-4) for the the network 172.16.0.0/16.
I can see all the OSPF neighbours on RTR-4. What is the reason?

When you shutdown the interface Fe0/1 on R3 you are breaking your OSPF Area 0 into two domains which is not good for OSPF. R4 still has a connection to Area 0 but this is now disconnected with the Area 0 on R2 and R1. While you will be seeing the required Summary LSA in R4 Area 1 database, R4 will not install it as it is itself an ABR (OSPF's loop prevention mechanism).

To avoid this OSPF Area 0 split in case of the R3 to R1 link failure (or shutdown) you can create a virtual-link between R4 and R2 through Area 1. This will keep your Area 0 contiguous and will allow R4 to install the required route.

Atif

Re: OSPF Query

Hi Atif,

Great answer. in face i have tried the same see below

when router is not ABR it install summary route...............

*Oct 25 13:13:39.231: OSPF: Add succeeded for summary route to 202.123.47.3/255.255.255.255, metric: 3, next-hop: GigabitEthernet1/0/10.10.10.2, area 1

when router is ABR it widraws this route

*Oct 25 13:19:54.571: OSPF-RIB-GLOBAL: Route delete succeeded 10.10.10.4/255.255.255.252 via 10.10.10.2 on GigabitEthernet1/0, source 10.10.10.2, return: 2.

This is because router expects this route to be injected from Area0 and not Area1

i must rate you

Regards

Mahesh

4 REPLIES
Cisco Employee

Re: OSPF Query

avilt wrote:

I am sorry, this is not the right OSPF topology. My test OSPF setup is shown in the attachment and I am particularly interested in the routing table on RTR-3 and RTR-4.
In a normal situation the routing table on both the routers are fine. However when I shutdown the interface Fe0/1 on RTR-3, there are no routes(on RTR-4) for the the network 172.16.0.0/16.
I can see all the OSPF neighbours on RTR-4. What is the reason?

When you shutdown the interface Fe0/1 on R3 you are breaking your OSPF Area 0 into two domains which is not good for OSPF. R4 still has a connection to Area 0 but this is now disconnected with the Area 0 on R2 and R1. While you will be seeing the required Summary LSA in R4 Area 1 database, R4 will not install it as it is itself an ABR (OSPF's loop prevention mechanism).

To avoid this OSPF Area 0 split in case of the R3 to R1 link failure (or shutdown) you can create a virtual-link between R4 and R2 through Area 1. This will keep your Area 0 contiguous and will allow R4 to install the required route.

Atif

Re: OSPF Query

Hi Atif,

Great answer. in face i have tried the same see below

when router is not ABR it install summary route...............

*Oct 25 13:13:39.231: OSPF: Add succeeded for summary route to 202.123.47.3/255.255.255.255, metric: 3, next-hop: GigabitEthernet1/0/10.10.10.2, area 1

when router is ABR it widraws this route

*Oct 25 13:19:54.571: OSPF-RIB-GLOBAL: Route delete succeeded 10.10.10.4/255.255.255.252 via 10.10.10.2 on GigabitEthernet1/0, source 10.10.10.2, return: 2.

This is because router expects this route to be injected from Area0 and not Area1

i must rate you

Regards

Mahesh

New Member

Re: OSPF Query

Now I remember reading somewhere that area 0 must be contiguous. Anyway thank you for the good technical explanation.

Cisco Employee

Re: OSPF Query

avilt wrote:

Now I remember reading somewhere that area 0 must be contiguous. Anyway thank you for the good technical explanation.

Yes Area 0 needs to be contiguous.

Mahesh,

Thank you for the feedback.

Atif

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