OSPF question - troubleshooting multiple paths & LSA's

[David Sharp]

Hi Everyone

I have an OSPF related question. OSPF is not my strong suit and I need to debug a routing issue.

Take the following situation for example:

 

 Assume that R1 is an ASBR redistributing routes between OSPF and BGP. R2 has a LAN subnet 10.20.20.0 /24. Everything else is Area 0.

What I want to debug is the routing path from R1 to 10.20.20.0. I can only see an external LSA, no local LSA. Shouldn't there also be an internal LSA for this subnet on R1 too since it's advertised via OSPF?

Another question. Say for example on R4, I want to see the paths to 10.20.20.0 /24. I only see one LSA. How can I figure out from "show ip ospf database" that there's two paths to this address?

What I'm trying to figure out essentially is whether or not the LSA's in OSPF contain information for each path, and how I can investigate routing paths within OSPF where there is more than one path for a particular subnet. 

Thanks everyone

[Joseph W. Doherty]

"I can only see an external LSA, no local LSA. Shouldn't there also be an internal LSA for this subnet on R1 too since it's advertised via OSPF?"

I think(?) there should be an internal LSA too. When you say you don't see if, you truly aren't seeing the LSA, or you're not seeing the internal in-area route?

"Say for example on R4, I want to see the paths to 10.20.20.0 /24. I only see one LSA. How can I figure out from "show ip ospf database" that there's two paths to this address?"

I don't think (?) you can. To "see" the other path, you would need to see the OSPF topology that's built before higher cost paths are pruned. The only time multiple paths are retained, in OSPF, is if they are equal cost.

Perhaps Peter Paluch will see your question and chime in.

[trfinkenstadt]

What do you see on R3 for routes on R1?  Just curious as to whether R1 and R2 are the same BGP ASN?

 I can see an issue come up if they are not as then R1 would prefer eBGP and redistribute that prefix into ospf...

--t