Re: NSSA Problem

[Acee Lindem <[e-mail]>]

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
>>
>
>
> __________________________________________________
> Do You Yahoo!?
> HotJobs - Search Thousands of New Jobs
> http://www.hotjobs.com
>
>


--
Acee