Ospf filtering routes

Unanswered Question
Aug 25th, 2007

hi everyone, i am running ospf with area 0 and area 1. There is an ASBR in area 0 which is injecting 2 routes and Now i want to advertise only in AREA 1. how can i do that ? i tried area 0 filter-list out but it filtered only the IA routes, but in my case i want to filter the specific external routes so how can i do that, i know i have to do something on the ABR in ARea 1 but i dont know how so kindly guide me.


I have this problem too.
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Mohamed Sobair Sat, 08/25/2007 - 01:01


This could be accomplished by issuing the command (Area 1 range subnet subnetmask) under the OSPF process of the ABR.


Mohamed Sobair

Harold Ritter Sat, 08/25/2007 - 04:26


There is only two ways you can filter out the external routes.

1) at the ASBR itself.

2) by configuring a stubby or not so stubby area. Note that in this case all external routes will be filtered out from the specific area.


Mohamed Sobair Sat, 08/25/2007 - 05:21

Hi Hritter,

Yes you are right, I was mistaken of saying command (Area x range) which will filter the summary-routes since the router in area0 is performing ABR/ASBR at the same time.

Apologize guys for misunderstanding the question carfully.


illusion_rox Sat, 08/25/2007 - 06:26

So wat u mean is that we cannot filter the specific routes on the ABR ?? is there any logic behind it or is it just cisco limitation ???

Mohamed Sobair Sat, 08/25/2007 - 22:48

Hi illusion,

As Hritter pointed out, The command (Area x range) where x is the area ID will only summarize the IA (Inter Area Routes), In your case, you can filter external routes using (Route-map) at the ASBR.

For example: you are injecting & respectively, if you wanna filter the second subnet from being entered the OSPF domain, you just create a route-map when you make redistribution as follows:

route-map (inject-to-ospf)

match ip address 10

Access-list 10 permit

router ospf 1

redistribute (protocol) route-map (inject-to-ospf)

* With the above you can acheive what you are looking for.

PlS rate if it helps


Mohamed Sobair

Mohamed Sobair Sat, 08/25/2007 - 23:02


Dont forget the key word (Subnets) when you make redistribution into OSPF, this will allow OSPF to recognize Classless Subnet.


Mohamed Sobair


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