ospf filtering

[Balajee Muggalla]

Hi,

    Is there a way to filter routes on per neighbor basis in ospf broadcast network? For Eg:, R1,R2 and R3 are in ospf area 1(nssa), R3 being ABR. If R3 receives two routes A and B from Area 0 and the requirement is :

      - R1 should receive route A(O IA) only,

      - R2 and future routers in this area should receive both the routes

      - Only R3 should be configured to do this

With the above needs, is there any solution?

- Balajee

[Jose Jara]

Hello Balajee,

OSPF filtering is tricky because it is a link-state protocol and all the routers in one area should have the same databases. The way to filter information in OSPF is via creating different types of Areas: stub, totally stub, etc.. or at the ABR's/ASBR's with specific commands. There are other ways, but these are the standard ones.

In your case, this would not be possible because LSA type 3 filtering (area 1 filter-list prefix in) could be done in the ABR- R3-  but this would affect all the routers in Area 1. Yoy may filter route B in R1 with distribute-list in or distance 255 but, be aware that this will filter the route in the RIB but not the LSA in the database and therefore there is a potential risk of blackholing the traffic in case router 1 is used as transit for prefix B.

Hope this helps,

Jose.