NSSA issue !

Unanswered Question
May 2nd, 2008

hi every one, suppose i have 3 areas, area 0,1,2. Area 2 is NSSA area. now i can see redistributed routes in area 1 and area 0, i can stop redist routes entering in area 1 by making it stub, but how can i prevent them from showing up in area 0 ?

1) i can use filter-list to filter these routes going from area 2 to area 0 right ?,

2) is there any other way ?

I have this problem too.
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Giuseppe Larosa Sat, 05/03/2008 - 01:25

Hello,

external routes cannot be filtered with the area filter-list command that can operate on LSA type 3 (summary LSA) on the ABR process of generation of LSA type 3 for each area they are connected to.

External routes in a NSSA area are sent in type 7 LSA data structures and they arrive at the ABRs (0, NSSA-area) where depending on the setting on the propagation bit P in the type 7 LSA they convert them in type 5 LSA or not.

So the key point on controlling if the an external route is propagated from an NSSA area to the backbone area is to control the P setting.

You cand do it on the NSSA ASBR when you create the route.

In this way NSSA provides the freedom to decide what routes are propagated and what are confined inside the NSSA area.

If you want to confine all the routes you can also use the no-redistribution option in the area 2 nssa command on the NSSA ASBR and ABR.

Hope to help

Giuseppe

Giuseppe Larosa Mon, 05/05/2008 - 06:52

Hello,

the way to have a redistributed route confined inside an NSSA area is to use the option no-advertise in the command summary-address in the router ospf configuration in the NSSA ASBR.

The no-redistribution option allows a ABR of an NSSA area to avoid to redistribute inside the NSSA area the external routes learned in the backbone area so it is useful for another effect.

As Edi says after you redistribute inside OSPF in area 0 or in a standard area there is way to filter the LSA type 5 of the external routes.

An NSSA area allows for it on a per route basis on the ASBR.

hope to help

Giuseppe

Edison Ortiz Sat, 05/03/2008 - 06:33

i can stop redist routes entering in area 1 by making it stub, but how can i prevent them from showing up in area 0 ?

You can filter routes during a redistribution by using a route-map+ACL combo from the originating routing protocol before entering OSPF.

Once the routes are brought into OSPF, you can't block those LSAs in Area 0 (the backbone). You can filter those LSAs from entering other areas (non-zero areas) but the backbone (area 0) must contain all the LSAs.

HTH,

__

Edison.

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