Subject: Re: [OSPF] issue for default route originate in NSSA

From: "Pat Murphy - (650)329-4044" <[email-masked]>
Date: Thu, 17 May 2007 07:23:32 -0700 (PDT)

Jin,

Sorry there was an omission in my previous post regarding

>RFC3101 2.5 Calculating type-7 as external routes (3) states:

>Else if the destination is a Type-7 default route (destination
>          ID = DefaultDestination) and one of the following is true,
>          then do nothing with this LSA and consider the next in the
>          list:

>            o  The calculating router is a border router and the LSA has
>               its P-bit clear.  Appendix E describes a technique
>               whereby an NSSA border router installs a Type-7 default
>               LSA without propagating it.
>
>            o  The calculating router is a border router and is
>               suppressing the import of summary routes as Type-3
>               summary-LSAs.
>            [NSSA]
>Suppose user perform following steps in scenario(there is one ABR and ASBR 
>in NSSA area):
>1)ASBR generate type-7 default route LSA to redistribute default route 
>from static route source.
>2)In ABR,Create type 7 default route LSA via command like 
>"default-route-originate" for this NSSA area.

>After step 2),the ABR calculation also should not consider type-7 default 
>route LSA redistribted by ASBRs?
>Am I missing something?please correct me.

The answer is still yes as stated in the previous post; but also all Type 7
default LSAs are ignored by NSSA ABRs when the no-summary option is
configured. Whenever an NSSA ABR has no-summary configured, its default LSA
has Type 3 so that it is preferred, in OSPF, by every one of the NSSA's non-ABR
routers. The NSSA's ABR type 3 default LSA insures that the NSSA's non-ABRs
must follow a summary default path through one of its ABRs when forwarding
traffic to inter-area destinations.

However, if one of the NSSA's non-ABRs has an installed non-OSPF default
route, like a static default route, enabling no-summary on the NSSA border
could adversely impact this non-ABR's ability to forward traffic to OSPF
inter-area destinations.

Pat

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