Subject: RE: [Isis-wg] Operation of P-toP links

From: Neal Castagnoli <[email-masked]>
Date: Wed, 2 May 2001 10:35:25 -0700

> If it detects that an LSP it holds is corrupt (bad checksum 
> in memory), 
> then it can treat it as expired and hence get a new one.

Treating corrupt LSPs as if they had expired causes a network wide purge of
them.  I could imagine a faulty router (maybe one with bad memory or bad
software) frequently purging many LSPs and so cause network wide disruption.

One way to "safely" recover is to reacquire the LSP database.  You need a
mechanism to cause the routers on the other side of WAN links to
reinitialize them, and so cause the LSP database to be synchronized again.
(This mechanism does not exist in 10589).

We added a state machine to the WAN hello processing that allowed one side
to notify the other that it was reinitializing the link, in NLSP, an ISIS
knock-off.  Among other things, this allowed NLSP to recover from corrupt
LSPs.

Recovering worked fine in the lab.  After corrupting an LSP by hand in the
database, the router would find the bad LSP during periodic LSP database
verification. The router then released the entire contents of the database
and reinitialized all the WAN links, thereby causing the LSPs to be
reacquired.

However, while it seems like a great idea to recover from corrupt LSPs, how
does the router know the extent of the damage?  It could be that other data
structures are now in an inconsistent state, some of which could cause
network wide disruption.  This is why the (simple) idea of "crashing the
router" makes a lot of sense.  It can help to isolate the problem to a
single node in the network (of course, it's best to prevent as many of these
problems as practical in the first place!).

> As I said, once a router has acknowledged an LSP it is its 
> responsibility 
> to preserve it in its memory. If it doesn't it should probably crash!
> 
>          Mike

  --Neal
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From: Tony Przygienda <[email-masked]>
Date: Wed, 02 May 2001 11:42:29 -0700

> software) frequently purging many LSPs and so cause network wide disruption.

any decent implementation does only flush its own LSPs these days. Flushing
other people LSPs has proven to be a bad idea.

> and reinitialized all the WAN links, thereby causing the LSPs to be

even simpler solutions to the problem exist given some cooperation from your
neighbor

            -- tony