Re: [PATCH net 0/2] vxlan: Set a large MTU on ovs-created vxlan devices

On Wed, Jan 6, 2016 at 12:59 PM, David Miller [off-list ref] wrote:

From: David Wragg <redacted>
Date: Wed,  6 Jan 2016 13:33:04 +0000

quoted

Prior to 4.3, openvswitch vxlan vports could transmit vxlan packets of
any size, constrained only by the ability to transmit the resulting
UDP packets.  4.3 introduced vxlan netdevs corresponding to vxlan
vports.  These netdevs have an MTU, which limits the size of a packet
that can be successfully vxlan-encapsulated.  The default value for
this MTU is 1500, which is awkwardly small, and leads to a conspicuous
change in behaviour for userspace.

These two patches set the MTU on openvswitch-crated vxlan devices to
be 65465 (the maximum IP packet size minus the vxlan-on-IPv6
overhead), effectively restoring the behaviour prior to 4.3.  In order
to accomplish this, the first patch removes the MTU constraint of 1500
for vxlan netdevs without an underlying device.

Is this really the right thing to do?  Won't we get a lot of fragmentation
by using such a large MTU, especially since you're making it the default
for OVS setups?

Things like path MTU discovery hinge strongly upon accurate MTU settings.
Otherwise they won't function properly.

At a minimum, I don't think this should be VXLAN specific. But I agree
that I'm not sure this is the right thing to do.
_______________________________________________
dev mailing list
dev@openvswitch.org
http://openvswitch.org/mailman/listinfo/dev