On Thu, Sep 14, 2017 at 12:10 PM, John Petrini <jpetr...@coredial.com> wrote:
> 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: neutron.conf:default:global_physnet_mtu ml2_conf.ini:ml2:path_mtu ml2_conf.ini:ml2:physical_network_mtus 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. Thanks, Curtis. > > Thank You, > > John Petrini > > > _______________________________________________ > Mailing list: http://lists.openstack.org/cgi-bin/mailman/listinfo/openstac > k > Post to : openst...@lists.openstack.org > Unsubscribe : http://lists.openstack.org/cgi-bin/mailman/listinfo/openstac > k > > -- Blog: serverascode.com
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