[Openstack-operators] How to deal with MTU issue on routers servicing vxlan tenant networks? (openstack-ansible with LXC containers)
davidy at funkypenguin.co.nz
Tue Dec 5 20:47:06 UTC 2017
I'm running Openstack Ocata (Deployed with openstack-ansible), with the
* Compute nodesrunning nova and neutron agent
* 2 x Controllersrunning neutronserver/agents in LXC containers (as
deployed by openstack-ansible playbooks)
* Underlying hosts have a single NIC (MTU 9000) with multiple VLAN
subinterafces, which in turn are connected to bridges br-vxlan, br-vlan,
I've encountered the following problem:
1. When I create in instance in a vxlan tenant network, without changing
any configuration files, the instance (linux default) assumes an MTU of
1500, but in reality only has an MTU of 1450 (because of the VXLAN
overhead). Instances cannot ping each other or their gateway (a neutron
router) with > 1450 MTU.
2. While I _could_ push an MTU of 1450 to my instances via DHCP, this is
(a) not always reliable depending on the guest OS, and (b) breaks
dockeron instances, which defaults to an MTU of 1500 for docker0
3. So, I attempted the configuration changes described at
increasing my global MTU to 1550 in neutron.conf / ml2_conf.ini, on the
compute nodes, and the neutron client & server LXC containers on the
controller, so that a default MTU of 1500 in my instances would always work.
4. The effect of step #3 above is that now my instances can communicate
with _each other_ at up to 1500 MTU, _but_ they still can't ping their
gateway (the neutron router) at anything over 1450 MTU.
5. When I examine my compute nodes (underlying host OS), I note that the
bridge "br-vxlan" contains the vlan subinterface (MTU 9000) plus a veth
interface for connectivity to the neutron-agents LXC container (e.g.
"04063403_eth10"). The veth interface has an MTU of 1500. The
corresponding interface within the neutron-agents LXC container (eth10)
also has an MTU of 1500.
6. Assuming that #5 is the cause of my MTU fault (i.e., a 1500-byte
packet from the instance over the tentant network = 1500+50=1550, can't
pass through the veth interface), I manually changed the veth interface
(and the corresponding interface within the LXC container) to MTU 1550.
7. Now I can pass packets from my instances to the neutron router as
large as 1468 bytes (previous limit was 1448), but still not the 1500
bytes I expected.
8. Increasing the MTU again (per #6 above) to 1600 makes no difference
to the result in #7 above.
So, I'm thinking I've missed something, and the most likely issue is the
definition of the LXC container (and veth interfaces) for neutron-agents
on the controller. I thought it was a simple fix (manually change MTU
per #6), but I'm baffled re why increasing MTU on the veth interfaces by
50 bytes only got me 20 bytes more overhead (1468), and even if this
_was_ the fix, it's obviously only temporary, so I wonder what is the
correct way to address the MTU issue under openstack-ansible?
Can anybody shed some light on this?
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