[Openstack-operators] Setting default MTU fro project networks?

Jonathan Proulx jon at csail.mit.edu
Fri Nov 4 17:34:17 UTC 2016

Hi All,

So long story short how do I get my ml2/ovs GRE tenant network to default to
MTU 9000 in Mitaka - or - get dhcp agents on on netork node to give
out different MTUs to different networks?

Seems between Kilo (my last release) and Mitaka (my  current production
world) Neutron got a lot cleverer about MTUs and teh simple
workarounds I had to make by jumbo frames go are now causing some
issues for newly created project networks.

Because I'm setting 'dhcp-option=26,9000' in /etc/neutron/dnsmasq.conf
everything get an MTU of 9000 inside the guest OS. I only *really*
care about this for our provider vlans, for project networks I only
care that they work.

CUrrently when a new project network is created it get an MTU of 1458
(1500 less GRE overhead) this is reflected in teh neutron DB and the
various virtual interfaces on the hypervisor and network node, but
DHCP configures inside the host to be 9000 and hilarity ensues.

I tried setting DEFAULT/global_physnet_mtu=9134 in neutron.conf and
ml2/path_mtu=9134 in ml2.ini (which is the actual MTU of L2 links),
agent/veth_mtu=9134 was previously set. I thought this would result in
virtualdevices large enough to pass the 9000 traffic but seems to have
made no difference.

I can kludge around by specifying MTU on network creation (or some
post facto DB hackery) but this isn't do able through my Horizon UI so
my users won't do it.


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