Open Stack

We are running Ocata and our MTU is not set to 1450.. since out external
provider is not running on a 9000.


Remo

On 9/14/17 11:10 AM, John Petrini 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?
>
> Thank You,
>
> John Petrini
>  
>
>
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