Subject: [Isis-wg] Clarification on ISIS SPF algorithm

From: Navaneeth K <[email-masked]>
Date: Thu, 20 Apr 2000 20:56:46 +0530

Hi,

This doubt is regarding the SPF algorithm mentioned in RFC 1195

The section 8 under STEP 0:

Says:

" Now add systems to which the local router does not have adjacencies, but 
which are mentioned in neighbouring pseudonode LSPs.The adjacency for such 
systems is set to that of the Designated router"

What does it exactly mean? When is possible that a router does not have 
adjacency to a system mentioned in a neighbour's pseudonode LSP?

Please clarify.

Thanx and regards
Navaneeth.

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From: Henk Smit <[email-masked]>
Date: Thu, 20 Apr 2000 10:24:50 -0700 (PDT)

Under buggy conditions on a LAN. Should never happen though.

  Another example might be when you configure your routers to model
 a Frame-relay, ATM or X25 cloud as a LAN. On those NBMA WAN technologies
 it is more likely that when some links fail, the resulting topology
 might be a partial mesh. Note that even if you do the trick that
 you quote, routing is still broken. IMHO on NBMA networks you need to
 configure all VCs as p2p links.

    Hope this helps,

         Henk.

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From: Mike Shand <[email-masked]>
Date: Tue, 25 Apr 2000 08:18:32 +0100

>  Under buggy conditions on a LAN. Should never happen though.

Strictly its not just buggy conditions, but it can happen for some period 
of time when a new router comes up etc. It might well acquire the adjacency 
to the DR and hence discover that certain other nodes are reachable via its 
pseudonode, before it has acquired the adjacencies to the nodes themselves. 
All this is doing is saying that if the pseudonode claims reachability, but 
you don't have an adjacency(yet), then you can always get packets to the 
node in question by forwarding them to the DR (since that MUST have an 
adjacency to the node, otherwise it wouldn't appear in its pseudonode LSP.

This was more of an issue with OSI, where the endsystem adjacencies could 
take a few minutes to be acquired in some cases (typically you run with 
quite long ESH timers to keep the traffic down from potentially thousands 
of Endsystems, and the soliciting mechanisms don't work for all 
implementations.)

So, bottom line is that it doesn't happen very often, and even if it does 
it doesn't happen for very long, but it does avoid the problem of not 
having an adjacency, and gets things working a little bit quicker.

         Mike