Re: Default LSA creation

[TAKENAGA Yoshinobu <[e-mail]>]

From: Acee Lindem <[e-mail]>
Subject: Re: Default LSA creation
Date: Fri, 11 Oct 2002 00:15:44 -0400

> TAKENAGA Yoshinobu wrote:
>
> > Hi,
> >
> > I have a question about creation of default lsa into stub areas.
> > (This question may be a FAQ, please suggest the point)
> >
> > Consider this network.
> >
> >    area 1   :  area 0
> >    (stub)   :     +--->>(AS extenal path)
> >             :     |
> >  |--R1--|   :  |--R4--|
> >  |      |--R2--|      |
> >  |          :         |
> >  |      |--R3--|      |
> >  |--R6--|   :  |--R5--|
> >             :
> >
> > Here, R2 and R3 are area border router.
> > R4 is AS boundary router.
> > All link have same cost.
> >
> > In such situation, R2 and R3 will create default Type-3 LSA into area 1.
> > R1 will select R2 as nexthop to the AS extenal path through R4.
> >
> > Now, the link state changes to this (R4 lose the link to N1).
> >
> >    area 1   :  area 0
> >    (stub)   :     +--->>(AS Extenal path)
> >             : N1  |
> >  |--R1--|   :  |XXR4--|
> >  |      |--R2--|      |
> >  |          :         |
> >  |      |--R3--|      |
> >  |--R6--|   :  |--R5--|
> >             :
> >
> > In such situation, R2 becomes a blackhole.
> >
> > What is the minimum requirement for R2 to create default Type-3 LSA
> > into stub areas?
> >   - having configuration of Area0
> >   - having active interface to Area0
> >   - having "Full" status neighbor in Area0
> >   - other??
>
>
> Hi Takenaga,
>
> The answer is other. The minimum requirement is that the
> router be an Area Border Router (ABR). Section 3.3 of RFC 2328 defines
> an ABR as a router that attaches to multiple areas. In the context of
> RFC 2328, area attachment requires at least one active interface in that
> area. Some implementations use other definitions of what constitutes
> an ABR (e.g., refer to draft-ietf-ospf-abr-alt-04.txt). However, using
> a different ABR definition doesn't handle all situations. Consider R6 and
> R3 in the figure below:
>
>
>      area 1   :  area 0
>      (stub)   :     +--->>(AS Extenal path)
>               : N1  |
>    |--R1--|   :  |--R4--|
>    |      |--R2--|      |
>    |          :         |
>    |      |--R3--|      |
>    |--R6--|   :  |--R5XX|
>
>
> In general, a good OSPF network design will minimize the possibility
> that the backbone will become partitioned.

Hi Acee,

Thank you for your response.

I understand. Some other good solutions(in such IDs) may have confused
me.  Now my question is cleard with your advice.

--
TAKENAGA Yoshinobu, Fujitsu.