Subject: Re: [Isis-wg] Regarding Area address option in non-pseudo LSPs

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Tue, 15 May 2001 11:34:54 -0400 (EDT)

Amudha asked:
	
	Hi,
	
	In section 7.3.5 of ISIS standard it is staes that:
	
	The Level 1 Link State PDU not generated on behalf of a pseudonode
	contains the following information in its variable length fields.
	- In the Area Addresses option – the set of manualAreaAd-dresses
	for this Intermediate System.
	
	In section 9.7 it states that:
	For non-pseudonode LSPs this option (Area addresses) shall always be 
pre-
	sent in the LSP with LSP number zero, and shall never be present in an 
LSP
	with non-zero LSP number. This option
	shall never be present in pseudonode LSPs.

	Aren't these statements contradictory or is my understanding wrong? Can
	anyone please help me out?
	
	Thanks and Regards,
	Amudha
	
Henk replied, and his reply was correct, but I think I can offer a more
tutorial explanation.

"Pseudonodes" are a way of making a LAN look like a node to the routing
algorithm, which makes the database and computation scale better than the
alternative, which would be considering all the nodes connected to the LAN
to be fully connected, and each reporting links to all the others. So the DR
"names" the LAN, and each node on that LAN just reports a single neighbor
(that LAN, i.e., the pseudonode). The DR, in addition to the LSP it
generates on behalf of itself (the "non-pseudonode LSP), generates an
LSP on behalf of the LAN (the "pseudonode LSP").

Sending manual area addresses around in LSPs is a way to ensure that all
the routers in the area agree on what the area addresses for that area are.
All the routers need to find out what area addresses have been configured
into other routers in that area. If area addresses were reported in ALL
LSPs (pseudonode and non-pseudonode), then that would just be duplicative
info, since the DR would report the same set in both its non-pseudonode
LSP and its pseudonode LSP. So the spec says to ONLY report it in non-pseudonode
LSPs.

Radia

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