[Openstack-operators] [Openstack] MTU on Provider Networks

Curtis serverascode at gmail.com
Fri Sep 15 12:59:47 UTC 2017

On Thu, Sep 14, 2017 at 12:10 PM, John Petrini <jpetrini at coredial.com>

> Hi List,
> We are running Mitaka and having an MTU issue. Instances that we launch on
> our provider network use Jumbo Frames (9000 MTU). There is a Layer2 link
> between the OpenStack switches and our Core. This link uses and MTU of
> 1500.
> Up until recently this MTU mismatch has not been an issue because none of
> our systems are sending large enough packets to cause a problem. Recently
> we've begun implementing a SIP device that sends very large packets,
> sometimes even over 9000 bytes and requires fragmentation.
> What we found in our troubleshooting is that when large packets originate
> from our network to an instance in OpenStack they are being fragmented (as
> expected). Once these packets reach the qbrXXXXXXXX-XX port iptables
> defragments the packet and forwards it to the tap interface unfragmented.
> If we set the MTU on the tap interface to 1500 it will refragment the
> packet before forwarding it to the instance.
> A similar issue happens the other direction. Large packets originating
> from the OpenStack instance are fragmented (we set the mtu of the interface
> in the instance to 1500 so this is expected) but again once the packets
> reach the qbr-XXXXXXXX-XX interface iptables defragments them again. If we
> set the MTU of the qvbXXXXXXXX-XX to 1500 the packet is refragmented.
> So long story short if we set the instance MTU to 1500 and the
> qbrXXXXXXXX-XX and qvbXXXXXXXX-XX ports on the compute node to 1500 MTU the
> packets remain fragmented and are able to traverse the network.
> So the question becomes can we modify the default MTU of our provider
> networks so that the instances created on this network receive a 1500 MTU
> from DHCP and the ports on the compute node are also configured to a 1500
> MTU?
> I've been looking at the following neutron config option in
> /etc/neutron/plugins/ml2/ml2_conf.ini:
> physical_network_mtus =physnet1:9000,providernet:9000
> Documentation on this setting is not very clear. Will adjusting this to
> 1500 for providernet accomplish what we need?

As far as I know there are 3 main options that one can use to set MTUs in
Neutron, and I use them to do things like ensure that my vxlan interfaces
have an MTU of 1500, as well as provider networks to 1500.

They are:


I usually set them all specifically (example: 9000, 1550, and
someprovider:1500). That said, I think only setting your providernet to
1500 would do what you want.


> Thank You,
> John Petrini
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