Re: MTU

[Vincent Bernat <bernat () luffy cx>]

 ❦ 22 juillet 2016 14:01 CEST, Baldur Norddahl <baldur.norddahl () gmail com> :

> Until now we have used the default of 1500 bytes. I now have a project were
> we peer directly with another small ISP. However we need a backup so we
> figured a GRE tunnel on a common IP transit carrier would work. We want to
> avoid the troubles you get by having an effective MTU smaller than 1500
> inside the tunnel, so the IP transit carrier agreed to configure a MTU of
> 9216.
>
> Obviously I only need to increase my MTU by the size of the GRE header. But
> I am thinking is there any reason not to go all in and ask every peer to go
> to whatever max MTU they can support? My own equipment will do MTU of 9600
> bytes.

You should always match the MTU of the remote end. So, if your transit
carrier configured 9126 on its side, you should do the same on
yours. There is no MTU discovery at the L2 layer: if you setup the MTU
of your interface at 9600 and you happen to route a 9500-byte packets,
it will be silently dropped by your transit carrier.
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