[c-nsp] MTU / BGP

[Mark Tinka]

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On 21/Jul/15 09:06, Gert Doering wrote:
>
> If you have a link where the L3 MTU is set higher than the L2 technology
> can handle, you're run into deep shit anyway - like, ISIS not coming up
> (due to padding to full MTU size), L3 packets spontaneously disappearing
> into a black hole (because your routers assume they can send them
> unfragmented...), etc.

Agree.

The problem is when you lease a Layer 2 network, as you mention below,
the MTU on your immediate connection to your provider may be different
from the one across their backbone that gets to your B-end, especially
if the provider does not have confidence that they have a consistent MTU
in their backbone.


>
>
>
> There's always the possibility for MTU differences in different parts of
> the network, like "this access segment here which hosts a xyz-box which
> can only do a MTU of 1900" - path MTU discovery will ensur that BGP
> from a box in this network to a box elsewhere on a MTU 9000 link will
> still nicely work (and fall down to a MSS of 1860 or so).

I agree.

The last time I tried MTU discovery for BGP, it broke badly. Maybe it
was the code - this was back in the days of SRC.

Never tried it again since.

Mark.
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