Tuesday, August 31, 2010

BSCI - Manipulating Routing updates

So, this evening I've been battling Distribution lists and the appropriate application of them and for some reason I just couldn't get that external EIGRP route to be filtered out when redistributing in to OSPF.

After a bit of reading around I've figured out I was approaching it from the wrong end. Assume you have R1, R2, and R3. R1 is running EIGRP, R2 is performing redistribution in to EIGRP and OSPF, and OSPF is running on R3.

My error was that I was working on R2 and wondering why it was that I had configured my access-list to deny my chosen route, applied the cmd - #distribution-list 1 out ospf 1 within the #router ospf 1 process, and nothing had happened.

The problem I was encountering was that in order for OSPF to properly calculate the shortest path, all the Link-State Databases through out your area must be synchronised. As such you can't simply deny your chosen network on the redistributing router as the network would not then be in synch.

The solution was to log on to R3, the router I wanted to have the route filtered from. Create the access list to deny the chosen route (then permit any - remember the implicite deny that would otherwise take affect). I then entered #distribution-list 1 in, from within the ospf routing process and job done! My desired route if filtered out and the rest remain.

Configuration:
R3(config)#access-list 4 deny   172.16.4.0 0.0.0.255
R3(config)#access-list 4 permit any
!
R3(config)#router ospf 1
R3(config-router)#network 10.10.0.0 0.0.255.255 area 0
R3(config-router)#network 192.168.12.0 0.0.0.255 area 0
R3(config-router)# distribute-list 4 in
!

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