Subject: Re: [rbridge] Proposed resolution of DRB election/MTU testing

From: mike shand <[email-masked]>
Date: Fri, 01 May 2009 09:42:33 +0100

Radia Perlman wrote:

Jeff Pickering wrote:

Radia,

Just for clarification, when determining
"The campus-wide minimum MTU size chosen is the smallest size "S"
reported in any LSP",
I am assuming you need to look all LSPs, not just "reachable via spf"
ones. Correct?
What happens when S shrinks and there are already extant LSPs with
size > old S?

Jeff

Yes, you'd have to look at all LSPs, look at the "S" contained in each
one, and choose the smallest S
as the campus-wide S. (though with the minimum value of S set to be an
architectural constant,
say 1470).

As for whether it's just "reachable via spf", I'd say yes, it should
only be ones reachable via spf.
Otherwise, one bad LSP from an about-to-die router can make the rest of
the campus have
to stick with a low value of S until that LSP times out.

The counter argument to that is of course that if the one currently
advertising the smallest value goes away for a short while everyone
would potentially change to a higher value for a short period and then
back. I suppose you could counter this by having a holding time on the
value you use, so it would have to be changed for say 15 mins before
you actuall actioned it.

But I think this whole thing is more trouble than its worth. We have
used a manual config of the value for the last 20 years and never had a
problem. I don't see adding all this extra complexity is worthwhile.

As for already extant LSP fragments that are now bigger than S -- good
question!

Obviously, the source of those LSPs
needs to reformat its LSP and flood the newer, smaller-fragmented LSP.

However, as to what to do with LSPs in your database before they get
reflooded from the source -- I'd
say time them out. In DECnet we had something where one router R could
decide to time out
some other router, R2's, LSP, by setting the time to zero and
reflooding.

Well that can still be done in IS-IS today, but it is frowned upon. The
only case is where a newly elected DIS is allowed to purge the old
pseudonode.

But then I remember
some sort of controversy about that (perhaps worrying about the security
of allowing someone
to do that, since there couldn't be a way of cryptographically
preventing a malicious router
from prematurely timing out someone else's LSP if it has to be possible
for a nonmalicious router
to legitimately time out someone else's LSP). Is that timing-out
mechanism still in IS-IS?

Yes.

But anyway, I don't have any strong opinions on what to do with LSP
fragments of size > S
that already exist. Seems like the possibilities are:
a) silently discard them
b) "noisily" time them out (i.e., flood them with age 0)
c) use them as if they're OK

Radia
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