Re: [Isis-wg] [rbridge] Why is MTU discovery important?

[Radia Perlman <Radia.Perlman at sun.com>]

OK. It's clear we should NOT pad Hellos.

It also seems like enough people would feel uncomfortable with not doing 
MTU discovery that we should make
sure there is a mechanism for that also.

So can we focus for a minute on the following question, "whether to make 
MTU size a forever constant,
or configurable in a campus", as described below.

The simplest option is to pick some minimum size, say 1450 bytes, put 
that in the spec now, and forever
say that TRILL Hellos and LSPs cannot be bigger than that.
MTU discovery would be done to ensure that
every link can handle 1450 bytes, and not use (not report in LSPs) links 
that can't handle that.

However, my preference is to have an optional TLV in the LSP that says 
"campus-wide MTU size". This TLV
would only be relevant if:
a) it is larger than 1450, and
b) it is in the LSP of the highest priority RBridge in the campus.

If we put that in, then we would be enabling TRILL campuses that support 
jumbo frames.

The rule is that if the highest priority RBridge says MTU size should be 
10000 bytes, then all RBridges would adjust
their buffer sizes, and test their adjacencies to ensure they could 
handle 10000 bytes, and if they don't
then stop reporting those links (just like they would with the simpler 
proposal if the links
couldn't support 1450).