OSPF NSSA

[Roel Reyes]

Hi All, 

Haven't tried this, what if i have NSSA Area and in that area i have ASBR1, ASBR2, ASBR3 and ASBR4

they create type 7 lsa from redistribution to the ABR to become type 5, are each ASBR receive every other external routes? or don't? will they only forward their own external routes to the ABR to become type 5?

Currently im running normal area, but routes are distributed via redistribute static meaning routes are external, im planning to limit external routes coming from other routers since it's not necessary for them to communicate. The only solution i came up is filter the routes (in filter) to each of my ASBR.

Thanks,

[cofee]

Comment to 1st part of question:

If all ASBRs belong to the same NSSA area then yes they would all receive type 7 lsa generated by an ASBR through redistribution and type 7 lsa/database (identical database within the area) would also be shared by the ABR of that NSSA area so it can be forwarded to other areas as type 5 lsa.

Comment to 2nd part:

You can use route-maps to filter prefixes for redistribution.

[Roel Reyes]

Hi,

Tell me if i get it corrert, asbrs on same nssa will be shared among each other, let say asbr1:10.0.0.0/8 and asbr2:11.0.0.0/8 they will share routes since they are on same nssa, it will not be block since nssa blocks only external routes type 5 not external routes type 7, only external routes coming from other igp or another nssa type 5 that was converted from type 7?

2nd
So best way is to filter routes going in from my routers.

[cofee]

Within the same NSSA area whatever prefixes you are restributing will be flooded to all other routers in that area as type 7 lsa. Now lets say there is a router in that NSSA area and you don't want that router to install a type 7 in its routing table that it's receving from an ASBR in the same areas, you can use a distributed list to filter that prefix and it won't be installed in the routing table of the router where you applied distributed list. Note - prefix will still be present in the database as they belong to the same area, but route won't get installed in the RIB because of distributed list.

Type 5 LSAs are not supported in area that's configured as NSSA, only ABR in NSSA can convert type 7 LSA into a type 5 LSA and flood them to other connected areas. NSSA routers can receive either a N2 or IA default route from the ABR of NSSA to reach type 5 routes in other areas or both IA and type 5 routes.