Subject: [OSPF] NSSA - Setting E bit in Router LSA

From: Rajesh Shetty <[email-masked]>
Date: Mon, 06 Oct 2008 15:40:42 +0530

Hi All,

As per NSSA RFC 3101, setting E bit in Router LSA:

ALL NSSA border routers set bit E in those router-LSAs originated into directly attached Type-5 capable areas. An NSSA's AS boundary routers also set bit E in their router-LSAs originated into the NSSA. 

Since NSSA border routers does not set E bit in router LSA which its originate for NSSA area, when internal routers in NSSA area does route calculation for Default Type-7 As-external LSA (Generated by NSSA border routers for inter-as reachability) ASBR cannot be found in originating NSSA. This will prevent internal routers from installing Default route there by preventing inter-as reachability.

I think NSSA border routers should set Ebit in those router-LSAs originated into directly attached Type-5 capable areas as well as to directly attached NSSA areas (When import summaries is set to enabled).

Correct me if I am missing any information.

Thanks

Rajesh

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From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Mon, 06 Oct 2008 07:54:04 -0700 (PDT)

Rajesh,

>ALL NSSA border routers set bit E in those router-LSAs
>originated into directly attached Type-5 capable areas. An
>NSSA's AS boundary routers also set bit E in their router-LSAs
>originated into the NSSA. 

>Since NSSA border routers does not set E bit in router LSA which
>its originate for NSSA area, when internal routers in NSSA area
>does route calculation for Default Type-7 As-external LSA
>(Generated by NSSA border routers for inter-as reachability)
>ASBR cannot be found in originating NSSA. This will prevent
>internal routers from installing Default route there by
>preventing inter-as reachability.

>I think NSSA border routers should set Ebit in those router-LSAs
>originated into directly attached Type-5 capable areas as well
>as to directly attached NSSA areas (When import summaries is
>set to enabled).

>From RFC 3101,

...Like OSPF ASBRs, an NSSA router indicates it is an NSSA ASBR by 
setting the E-bit in its router-LSA.

I believe RFC 3101 makes no exception to this statement. Hence 
even an NSSA border router that originates any Type-7 LSA 
(including the Type 7 default LSA) MUST set the E-bit in its 
router-LSA as it is also an ASBR. However if an NSSA border router 
does not originate any Type 7 LSAs into an NSSA (even in RFC 1587 
when importing summary LSAs the origination of a Type 7 default 
LSA was optional) then it may optionally not set the E-bit in its 
router-LSA for that NSSA. I don't see a problem with this. Summary 
routes are not Type 7 LSAs and, when installed, are preferred over 
Type 7 LSA paths. One might do this if they want default traffic 
to follow an NSSA originated Type 7 default LSA possibly for 
security reasons.

Pat