On 22 January 2014 00:00, Robert Collins <[email protected]> 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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