[ovs-discuss] mtu setting lost

Jesse Gross jesse at nicira.com
Tue Nov 12 00:54:21 UTC 2013


On Mon, Nov 11, 2013 at 5:39 PM, Andreas Pflug
<pgadmin at pse-consulting.de> wrote:
> I'm running ovs (1.4.2) for xen bridging from debian 7.1.
>
> The physical interface is configured to the max possible MTU of 9216,
> because there's one ovs port configured for san access with a VLAN tag
> and MTU of 9190 that's used by open-iscsi initiator exclusively. The
> machine's main/management interface is configured to default MTU 1500.
>
> If I start a VM, its VIFs will be created with MTU=1500. When they are
> added to the ovs bridge, subsequently all non-physical interfaces will
> be reset to MTU=1500, including the (for common network traffic unused)
> san port. The same will happen if I further reduce one interface MTU
> manually; its setting will propagate to all ports. This appears
> unexpected to me and is undesirable.

The Ethernet bridging spec requires all interfaces on a single L2
domain to have the same MTU, which is why OVS is automatically
adjusting the MTU of the interfaces that it has control over. If there
truly is no overlap between domains then you could use separate OVS
bridges.



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