Re: NSSA Problem

[Amit Srivastava <[e-mail]>]

Hi Acee,
      I am sorry but i would not agree to your comment
See when multiple ABRs are there each of them would
have given router LSA in both the Area and when it
gets an AS-External LSA it can simply send the LSA to
other Area and will also give summary LSA(Type 4) for
the LSA. Now the router who will receive this LSA
would just do the SPF calculate the shortest ABR and
forward the packet to that ABR.
        That is why i am not able to appriciate the
Must condition of having forwarding address in the
type 7 LSAs.
Regards
Amit

--- Acee Lindem <[e-mail]> wrote:
> Amit Srivastava wrote:
>
> > Hi Pat and Acee,
> >       See even if the forwarding address is not
> set in
> > the originating AS-external LSA(in the NSSA) the
> ABR
> > can know about the router which has originated the
> LSA
> > by its router id. So why they are imphasising on
> > setting of the forwarding address!!
> > I am still in the same state of confision plz
> help!!
>
>
> Amit,
>
> If the NSSA has multiple ABRs and there is no
> forwarding
> address, routers outside the NSSA will always go
> through
> the translating ABR independent of whether or not it
> is the shortest path.
>
> Thanks,
> Acee
>
>
> > Regards
> > Amit
> >
> >
> > --- "Pat Murphy - [phone]"
> > <[e-mail]> wrote:
> >
> >>Amit,
> >>
> >>
> >>>           I can understand the point mentioned
> in
> >>>RFC 2328 about the forwarding address but in this
> >>>
> >>case
> >>
> >>>I am not able to appreciate the MUST condition
> the
> >>>RFC(with Exception when u start aggregating
> >>>
> >>multiple
> >>
> >>>routes).
> >>>
> >>It is true that type 7 LSAs that are aggregated
> into
> >>a single
> >>Type 5 LSA with a 0.0.0.0 forwarding address do
> not
> >>need a
> >>computed forwarding address. But usually an NSSA's
> >>ASBR has no
> >>way of knowing whether or not its self-originated
> >>Type-7 LSAs
> >>are aggregated by the NSSA's ABRs during
> >>translation. Without
> >>providing configuration to the contrary, the
> NSSA's
> >>ASBRs MUST
> >>assume their Type 7 LSAs (with the P-bit set) will
> >>not be
> >>aggregated during translation and therefore MUST
> >>compute a
> >>non-zero forwarding address.
> >>
> >>I don't see much value in complicating the NSSA
> spec
> >>by
> >>softening this requirement for Type 7 range
> >>aggregations or
> >>for the case of an NSSA with just a single ABR.
> Its
> >>hard to
> >>imagine any IPv4 application where the forwarding
> >>address
> >>cannot be set (IPv6ers care to jump in???). Note
> >>that when the
> >>P-bit is clear, the new NSSA spec does allow a
> Type
> >>7 LSA to
> >>have a 0.0.0.0 forwarding address.
> >>
> >>
> >>>An ABR for an NSSA does not generate type 4
> >>>
> >>summaries for
> >>
> >>>ASBRs within the NSSA.
> >>>
> >>This is because all type 5 LSA translations of
> type
> >>7 LSAs are
> >>originated by NSSA ABRs, not NSSA ASBRs.
> >>
> >>
> >>>Hence a forwarding address is
> >>>required to compute the best path to the prefix
> >>>advertised in the translated LSA.
> >>>
> >>If the NSSA's ABR translators used a 0.0.0.0
> >>forwarding
> >>address in their Type 5 LSA translations, these
> Type
> >>5 LSA
> >>translations would cause packets to be forwarded
> >>directly
> >>through their originating ABR rather than via a
> >>potentially
> >>more preferred path through a different NSSA ABR.
> >>Note that
> >>this is always the case when type 7 ranges
> aggregate
> >>Type 7
> >>LSAs into a Type 5 LSA with a 0.0.0.0 forwarding
> >>address.
> >>
> >>Pat
> >>
> >
> >
> > __________________________________________________
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> >
>
>
> --
> Acee


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