Subject: [Isis-wg] Adjacencies and LSP Numbers

From: Talmon_Reiss <[email-masked]>
Date: Wed, 9 May 2001 15:30:09 +0300

Hi

In 10589, 7.3.4.4, it says "Once a particular adjacency has been assigned to
a particular LSP Number, it is desirable that it not be moved to another LSP
Number." 

As I understood it, the LSP Number was supposd to be used when a LS PDU was
too large to fit in a single LSP (the  LSP Number 0 then has a special
significance wrt to the DB overload bit etc). 

Could someone please clarify this concept of the link between an an
adjacency and a LSP Number?

Thanks
Talmon

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From: mike shand <[email-masked]>
Date: Wed, 09 May 2001 13:47:46 +0100

>significance wrt to the DB overload bit etc).

Correct

>adjacency and a LSP Number?

The point is that if you move an adjacency between one LSP and another, 
there are race conditions which can result in momentary loss of 
information. SO, for example if the new LSP without the adjacency 
propagates before the new LSP with the adjacency, then SPF may run and 
compute the topology without the link, then an SPF hold-down prevents SPF 
from being run again, so the new LSP doesn't get taken account of.

So it is DESIRABLE not to arbitrarily move a link from one LSP to another.

         Mike

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From: Jeff Learman <[email-masked]>
Date: Wed, 09 May 2001 09:32:57 -0400

>significance wrt to the DB overload bit etc). 

You are correct.  However, once an adjacency is assigned to an LSP fragment,
it is "polite" to try to leave it in that same fragment.  This avoids
route flaps that can happen just because a link moved from one
fragment to another, and a router receiving these LSPs gets the new
LSP fragment without the link first and runs SPF -- only to receive the
next new fragment where the link is up.