Subject: Re: NSSA Question

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Mon, 23 Sep 2002 18:16:51 -0700

Mani,

> My question is, if R1 in Area1 receives type-4 Summary LSA for
> NSSA ASBR and able to reach the ASBR through inter-area path,
> the external destination must be reachable without advertising
> type-4 LSA for R3 into area1. Is my understanding correct ....

No. Regarding

>                                    ASBR
>       -----     -----     -----    -----     -----
>      |     |   |     |   |     |  |     |   |     |
>      | R1  |   | R2  |   |  R3 |  |  R4 |   | R5  |
>      |     |   |     |   |     |  |     |   |     |
>       -----     -----     -----    -----     -----
>           |     |   |      |  |     |  |     |
>            -----     ------    -----    -----
>            AREA1     AREA 0    AREA2    RIP Network
>                                 NSSA

there should not be a Type 4 summary LSA for R4 in either R1 or R2's
LSDB. However NSSA implementations may be a little sloppy in suppressing
these, since, their existence, while useless, is harmless to the
forwarding process. For R1 to install routes to networks learned by R4
from R5 via RIP, it must have an installed route to R3 (installed by
calculating a route to R3 during the processing of its Type 4 summary
LSA, which was originated by R2), plus installed routes to their Type 5
LSA forwarding addresses (installed during the SPF calculation as
opposed to the processing of Type 4 summary LSAs) that were copied from
R4's Type 7 LSAs during their translation by R3 to Type 5 LSAs. Any Type
4 summary LSA for R4 will never factor in the installation process of
these Type 5 LSAs originated by R3.

Type 4 summary LSAs for NSSA ASBRs do clutter the LSDBs and routing
tables of Area 0 and Area 1 with useless LSAs and routing table entries
respectivelyfor these NSSA ASBRs. Implementations should not originate
Type 4 summary LSAs for NSSA ASBRs. If they do, complain.

Pat