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

[Donald Eastlake <d3e3e3 at gmail.com>]

Keep in mind that all IS-IS messages are LSAP encoded so they are
limited to the classic Ethernet MTU, possibly plus VLAN and/or other
tags. So if, for example, you wanted to be sure you could correctly
handle Fiber Channel over Ethernet frames, which are bigger than that,
there is no way for padded Hellos to help, since they can't be padded
up to FCoE frame size.

Donald

On Tue, Apr 7, 2009 at 5:29 PM, Dinesh G Dutt <ddutt at cisco.com> wrote:
> Radia,
>
> What I don't understand is how L3 IS-IS solves this problem. Consider a
> trivial topology where three routers are connected thus:
>             R1 --- R2 --- R3
>
> Let's say the link between R1 and R2 has an MTU, M1, and the link
> between R2 and R3 has an MTU, M2. Let M1 > M2. Even with padded Hellos,
> how is the problem of R1 sending an LSP that exceeds M2 (but not M1)
> handled ?
>
> Dinesh
> Radia Perlman wrote:
>> 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).
>>
>>
>>
>>
>> _______________________________________________
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>> rbridge at postel.org
>> http://mailman.postel.org/mailman/listinfo/rbridge
>>
>>
>
> --
> We make our world significant by the courage of our questions and by
> the depth of our answers.                               - Carl Sagan
>
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