OSPF Admin distance and Multi area path selection

telecodi · ‎05-31-2007

Hi from Barcelona

I need help resolving an issue with OSPF Admin distances. See attached file to see my simple topology.

The problem is in DATACENTER-2. In a normal situation (all links up), the DATACENTER-2 routing table shows a route to REMOTESITE via the backup tunnel even if the DATACENTER-1 is populating the sem route with less cost. It's that way because for DATACENTER-2, the path trough the backu tunnel is an INTRA-area router whereas the path through DATACENTER-1 is an INTER-area router and OSPF will always choose first the INTRA area routes. I posted this question yesterday and the answer I was to modify, in DATACENTER-2, the administrative distance of routes received from REMOTESITE.

I have done it with the order distance 140 172.16.248.1 0.0.0.0 and now it happens:

- if the backup tunnel is doen, the DATACENTER-2 installs the route to REMOTESITE via DATACENTER-1 with [110/30], and it's correct. When I restore the backup tunnel, the route remains on teh routing table

- but if the point-to-point link goes down, the DATACENTER-2 routing table shows a route to REMOTESITE with [140/160]; also correct. But when the ptp comes up again, the route via the backup tunnel remains in the rounting table event DATACENTER-1 knwons the same route but a bit better [110/20]

Any help will be very apreciated

Dani

sundar.palaniappan · ‎05-31-2007

Dani,

I don't have access to a lab at this time to test. But, I have a feeling the reason why DATACENTER-1 continues to use the route via DATACENTER-2 after the p-t-p link comes back up is because OSPF prefers an aged route over a newer route. Can you try modifying the distance, to a higher value, on DATACENTER-1 router as well for the route learned via DATACENTER-2 and check the outcome.

If you are still having problems I shall try to lab it up later today and post a solution for you.

HTH

Sundar

telecodi · ‎05-31-2007

Hi Sundar

I have increased the distance value to 140 but with no result. I have attached 2 files, one with all 3 configs and one with show ip route on every router after shut/no shut down the ptp link and the backup tunnel in different order (the ptp link is simulated by Tunnel 5 on DATACENTER-1)

Thanks a lot for your time

Dani

sundar.palaniappan · ‎05-31-2007

Dani,

I didn't even care to lab it up because the very first output in your attachement (seen below) indicates the DATACENTER-2 router doesn't care about the distance and it always prefers the intra-area over inter-area route when all links are up.

##########################################

All links UP (after restarting all routers)

##########################################

Datacenter-1:

O 172.16.248.0/24 [110/110] via 172.16.255.233, Tunnel5

Datacenter-2:

O 172.16.248.0/24 [140/210] via 172.16.255.245, Tunnel6

If you want DATACENTER-2 router to serve only as a backup router then can't you influence the traffic coming from the DC LAN and remote sites (if any) to route all the traffic over to DATACENTER-1 router. This way no traffic would come into DATACENTER-2 when DATACENTER-1 is up and you don't have to worry about what it's routing table looks like.

HTH

Sundar

telecodi · ‎05-31-2007

Hi Sundar

I could use HSRP and make DC-1 and DC-2 to share the the DC-LAN's default-gateway, with DC-1 having the highest HSRP priority. It would work but it's not optim because DC-LAN and DC-LAN are on the same building whereas DC-1 is hosted in COLT-Telecom, 20 Km faraway. So, when a DC-LAN client wants to access the Internet, the traffic would travel 2 times across the layer-2 link that connects both DC before reaching the Internet. That link is very expensive and is overloaded. We could also implement QoS but it isn't the question.

I'm trying to implement network easy to undertand and, therefore, to troubleshoot. There is no solution, isn't it?

Dani

sundar.palaniappan · ‎06-01-2007

The only other thing I can think of is you can use static or reliable static route using object tracking on DATACENTER-2 router to route all traffic to the remote site LAN via DATACENTER-1 router. However, this solution isn't greatly scalable if you have several remotes that you want to route traffic the same way and the LAN subnets aren't contiguous to be summarized.

HTH

Sundar

telecodi · ‎06-04-2007

Hi Sundar

as you suspected, I have several remote sites so none of the solutions is perfect. Anyway, I would like to thank you for your time and interest helping me.

THANKS !

Dani