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

[Les Ginsberg (ginsberg)]

> -----Original Message-----
> From: James Carlson [mailto:james.d.carlson at Sun.COM]
> Sent: Wednesday, April 15, 2009 6:07 AM
> To: Sadler, Jonathan B.
> Cc: Silvano Gai (sgai); Les Ginsberg (ginsberg); Radia Perlman; isis-
> wg at ietf.org; TRILL/RBridge Working Group
> Subject: Re: [Isis-wg] [rbridge] Why is MTU discovery important?
> 
> Sadler, Jonathan B. writes:
> > So why did James Carlson's implementation break when an 802.1ad
Ethernet
> bridge (i.e. without .1Q extensions) was put between two RBridges?
Thats
> what started this whole discussion...
> 
> It's because slapping an extra header on top of a 1500 octet message
> tends to make it somewhat larger than 1500 octets.  :-/
> 
> Right now, TRILL headers are effectively part of the payload, as far
> as ordinary bridges are concerned, and are subject to the MTU
> restrictions.  (Unlike "overhead" information between bridges, like
> VLAN tags.)  If we could have access ports agree to use 1478 instead,
> we'd be able to meet a 1500 restriction between RBridges.
> 
> Of course, that answer is just infeasible.

Indeed - the point being that whatever bad things that happen to large
size L2IS-IS PDUs will also happen to large size data PDUs that are
TRILL encapsulated.

So the question I would like folks to consider is whether there is a
point in trying to utilize a LAN on which only small data packets can be
guaranteed to be forwarded successfully? If the answer is "no" then I
think the problem that needs to be solved is constrained and the set of
appropriate solutions becomes more easily defined.

   Les

> 
> In testing, I've found that some devices limit to 1500 strictly.  Some
> are lax up to about 1536.  Despite what standards may say about it,
> there's a lot of odd stuff out in the field.
> 
> --
> James Carlson, Solaris Networking
<james.d.carlson at sun.com>
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