OSPF default route origination from multiple router.

Unanswered Question
Sep 2nd, 2008

Hello... I have 5 router in my ospf domain RTR1,RTR2,RTR3,RTR4,RTR5. router RTR1 is connected to internet and originating Default route for the domain. But Now I Have another router RTR6 also connected to internet . Now I want RTR1 and RTR6 will originate difault route for the ospf domain and RTR2,RTR3 will recive default route from RTR1 and RTR4,RTR5 will receive default route from New Router RTR6. Is that possible. If possible pls help me how I can do This.

I have this problem too.
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Giuseppe Larosa Tue, 09/02/2008 - 10:36

Hello Asim,

if you have R1 and R6 to generate a default route of type E1 the routers in the OSPF domain will receive both LSA type 5 and will use the one with a lower overall metric:

O E1 : seed metric + metric to reach ASBR

to get it just add metric-type 1 on the default-originate command inside router ospf process on R1 and R6.

Depending on your topologies you can have different results: half routers going through R1, some routers installing both routes if they have equal cost from their own point of view.

You cannot however filter LSA type 5 in OSPF: you can use distribute-list to avoid to have a prefix installed in the local routing table but this will not stop LSA flooding in OSPF domain.

You can also play with different seed metric to get a better result if needed.

Hope to help

Giuseppe

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