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Re: [Isis-wg] [rbridge] Why is MTU discovery important?
Radia,
The minimum size packet is what I proposed and I think it is the easiest
solution to go forward. We should also allow for higher size packet if
there is a mechanism to safely enable that.
Now it is not because somebody (the highest priority Rbridge) to tell
you what to do that you have to blindly follow, but if everybody
announce that jumbo value then you can safely move to that value.
So here what we can do:
Put an originatingLSPBufferSize in a TLV when generating LSP and (ISO
10589:2002) already recommend that. The default value would be 1450
otherwise it is set to the configured value.
An Rbridge can start generating a non-default size X LSP if all the
reachable Rbridge has announce at least a value >= X, in other word the
non-default size LSP is set to the minimum of all announced LSP size.
Note that for a non-default size LSP, we rely on the administrator's
configuration and we don't actually check the MTU path. If later there
is a mechanism to actually check the MTU of a Rbridge link, then
originatingLSPBufferSize could be set to the minimum MTU size of all
local Rbridge's link. This will guarantee that the LSP size will always
be set to the minimum MTU size of any link.
Thanks
Sina
>
> 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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