Subject: [Isis-wg] draft-ietf-isis-restart-03.txt - RA and CSNP

From: Jeff Parker <[email-masked]>
Date: Wed, 9 Jul 2003 13:53:14 -0400

Mike & Les -
	Wondering about the interaction between the RA Hello and CSNPs on
LAN
	I'm looking at draft-ietf-isis-restart-03.txt, page 4, section
4.3.1.

	You suggest that an assisting IS first send an RA hello, and then
decide if it should send CSNPs.  What is the purpose of the RA without
CSNPs?  Is this to deal with the possibility that the assisting IS makes the
wrong decision, and does not send the CSNPs when everyone else thinks it
should?  

Or, to look at it from the other side:
	Must the restarting IS wait for a CSNP set from a peer who sends an
RA, or will any full set of CSNPs do?

- jeff parker

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From: Les Ginsberg <[email-masked]>
Date: Wed, 09 Jul 2003 12:30:38 -0700

>wrong decision, and does not send the CSNPs when everyone else thinks it

The purpose of the RA is independent of whether the sender will also send 
CSNPs or not. Receipt of an IIH/RA by the restarting router is used to 
determine - based on the remaining hold time - how long before the 
adjacency will go down. This is used to adjust the upper bound for T3 - 
which determines when we would begin to flood our own LSPs w OL bit set. 
Section 4.4.1.1 says on page 11:

"If the timer T3 expires before all the T2 timers have expired or
    been cancelled, this indicates that the synchronization process is
    taking longer than minimum holding time of the neighbors. The
    router's own LSP(s) for levels which have not yet completed their
    first SPF computation are then flooded with the overload bit set to
    indicate that the router's LSPDB is not yet synchronized (and other
    routers should therefore not compute routes through this router).
    Normal operation of the update process resumes and the local
    forwarding tables are updated."

>RA, or will any full set of CSNPs do?

This is an interesting point. Receipt of a full set of CSNPs is on a per 
interface basis. Clearly a set of CSNPs must come from a single source - 
and they should NOT come from a router which is itself restarting since 
that router does not yet have a full set of LSPs. Of the set of candidates 
on the LAN, it does not really matter which of the eligible routers sends 
the CSNPs - though we expect that the algorithm defined in 4.2.1c will 
result in only one router on the LAN sending CSNPs.

Version 4 of the draft will have some language clarifying this point. 
Inasmuch as we missed the deadline for draft submission, the new version 
will become available shortly after the upcoming meeting.

    Les