OSPF and virtual-link configuration

[fabio.marino]

Hi to all, i have a doubt concerning the virtual-link configuration, in the context of discontinuos ospf area.

In order to configure a virtual-link we need to use the router-id ip address.

R0<-----AS 0--------> R1 <------AS 16-------> R2<------AS 15------->R3

Using the above topology, the transit area is the AS 16.

Suppose that on router R2, the router-id is a loopback interface belongs to the OSPF AS 15.

Can i use that interface (belonging to AS 15) in order to create the virtual link (using as transit area the AS 16), or i have to issue the router-id command on router R2?

May be is not a problem but i have this doubt, and i'm testing that scenario on GNS3 but i would like to have also your opinion.

Thanks.

[Peter Paluch]

Hello Fabio,

I love your question, this would make a really good exam topic for my students

Note that in the virtual link configuration, you are referring to the other router's OSPF Router ID (RID). The RID is a unique number for a router as a whole - it is not a network, it is not an IP address, and it does not belong to any area. It is simply a 4B-long number that uniquely identifies a router, nothing more.

So it is absolutely irrelevant what area does the particular loopback on R2 belong into. Note that the interface from which the RID was initialized does not even have to be advertised in OSPF at all.

You can safely put the loobpack on R2 to any area you want, or you may choose to not to advertise it in OSPF at all. With virtual links, you simply reference the other ABR's RID regardlessly where it came from.

Best regards,

Peter

[fabio.marino]

Thanks Peter,

you are right but one more question is rising up.

I tried to configure the virtual-link, but know i want to test if it's working.

Is there some method? I mean i can't see R3 route on R0 and i wolud like to know if the virtual link that i created is working correctly

[Peter Paluch · Accepted solution]

Hi Fabio,

I assume you have configured the virtual link between R1 and R2. I would first suggest using the command show ip ospf virtual-link on both routers to see whether the virtual link has come up (also, you should have been informed that the R1 and R2 established a new adjacency over the virtual link - see the show ip ospf neighbor). If that virtual link is present then you should be able to see R3's networks on all routers in the topology.

Best regards,

Peter

[fabio.marino]

Many thanks Peter!!!

[Peter Paluch]

Fabio,

You are heartily welcome!

Best regards,

Peter