[ovs-discuss] Small 802.1q-prepended packets not getting through to VM

Steinar H. Gunderson steinar-ovs at gunderson.no
Thu May 30 08:18:29 UTC 2019

On Wed, May 29, 2019 at 07:04:48PM +0200, Steinar H. Gunderson wrote:
> I have an appliance VM (Cisco vWLC) in libvirt/KVM, connected to an Open
> vSwitch bridge through a virtio NIC. For untagged packets, this works fine.

An update on this:

I've figured out that the untagged packets actually _don't_ work fine;
they have the same issue as the tagged packets. So ARPs don't go through,
neither on untagged nor tagged interfaces.

However, I happen to be on a network where my local gateway proxyarps, so it
would respond to ARPs for my VM's IP address, and traffic would flow from the
host to the VM through the physical switch (which would add proper Ethernet
padding) and it would work. That's the reason why I assumed only the
802.1q-tagged VLAN had problems.

I've tried changing the NICs in the VM definition from virtio to e1000. This
seemingly fixes communication from the host to the management plane in the VM
(I no longer need to go through the physical switch), but small packets to
the wireless clients behind it are still eaten. (As I understand it, the
wireless controller does all of its non-management packet processing in user
space, so it's not surprising that the kernel driver could accept something
illegal that the actual controller would not.)

/* Steinar */
Homepage: https://www.sesse.net/

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