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On 22 January 2014 00:00, Robert Collins <robertc at robertcollins.net> wrote:

> I think dropping frames that can't be forwarded is entirely sane - at
>
a guess it's what a physical ethernet switch would do if you try to
> send a 1600 byte frame (on a non-jumbo-frame switched network) - but
> perhaps there is an actual standard for this we could follow?
>

Speaking from bitter experience, if you've misconfigured your switch so
that it's dropping packets for this reason, you will have a period of hair
tearing out to solve the problem before you work it out.  Believe me, been
there, rabbit messages that don't turn up because they're the first ones
that were too big are not a helpful diagnostic indicator.

Getting the MTU *right* on all hosts seems to be key to keeping your hair
attached to your head for a little longer.  Hence the DHCP suggestion to
set it to the right value.

> (c) we require Neutron plugins to work out the MTU, which for
> > any encap except VLAN is (host interface MTU - header size).
>
> do you mean tunnel wrap overheads? (What if a particular tunnel has a
> trailer.. crazy talk I know).
>

Yup, basically.  Unfortunately, thinking about this a bit more, you can't
easily be certain what the max packet size allowed in a GRE tunnel is going
to be, because you don't know which interface it's going over (or what's
between), but to a certain extent we can use config items to fix what we
can't discover.

-- 
Ian.
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