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List: ms-ospf
Subject: Re: [OSPF]
From: Acee Lindem <acee () redback ! com>
Date: 2008-07-24 14:50:27
Message-ID: D7975380-5550-4CB0-9073-251A318AB816 () redback ! com
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Hi Guoqiang,
On Jul 24, 2008, at 9:38 AM, guoqiang 03022 wrote:
> Hi, Acee
>
> Thanks for your answer, but i still have a question about the
> flushing algorithm.
>
> As you say, in most cases, a type-7 prefix is imported by one NSSA
> ASBR, so there is only one ABR will translate the prefix to Type-5
> LSA, even though there area more than one ABRs have the translation
> capability.
> If so, what's the purpose of the flushing optimization?
>
> As per RFC3101, 3.3 Flushing Translated Type-7 LSAs:
>
> --
> Conversely
> -- Type-5 LSAs generated by translating Type-7 LSAs are not
> immediately
> -- flushed, but are allowed to remain in the OSPF routing domain
> as if
> -- the originator is still an elected translator. This minimizes
> the
> -- flushing and flooding impact on the transit topology of an
> NSSA that
> -- changes its translators frequently.
>
> If the two ABRs are going to translate the same Type-7 LSA, then
> one must flush its translated Type-5 LSA, or
> the other will not do the translation. So the Type-5 LSAs area
> still flushed and generated if the NSSA changes its trnslators
> freuently.
> Is there something I misunderstand?
If you've delayed purging your type-5 LSAs until a new translator has
taken over and originated functionally equivalent translated type 5
LSAs, then you've achieved your intended purpose. However, I can see
a race condition here where the new translator has a lower router ID
than the previous one. This probably should have been mentioned in
RFC 3101.
Thanks,
Acee
>
> Thanks and Regards
> guoqiang
>
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