[ovs-discuss] can not create device problem

Jesse Gross jesse at nicira.com
Fri Jul 9 17:42:21 PDT 2010

On Thu, Jul 8, 2010 at 10:56 PM, Yan <yan.p.bjtu at gmail.com> wrote:

> Hi,
> I found another problem about the tunnel, its about the gre-dev's mtu in
> OVS.
> Thanks a lot for your suggestions and GRE of OVS works fine , but when I
> tried to use it on a special path, it stopped working
> The special path is actually a physical tunnel network path so when I again
> use OVS's GRE over it, I think maybe the MTU is too big for OVS GRE port do
> deal with it properly.When I forwarded the packet to the GRE port in OVS, it
> couldn't get out from the destination side.(Between the two sides the
> physical path is a tunnel network already).

There are several places that you could be losing packets due to MTU:

1. The GRE tunnel ports have a 1500 byte MTU.  If your packets are larger
than this they will be dropped and it will be logged in the kernel message

2.  By default we do path MTU discovery inside of tunnel ports.  If you are
sending packets that are larger than the actual MTU of the tunnel (the path
MTU of the physical path minus the tunnel overhead) with the "don't
fragment" bit set and ignore the returned ICMP message no packets will get

3.  We also do path MTU discovery on the outside of the tunnel packets.  If
the tunnel packets are larger than the MTU of some link on the network and
you have a firewall that is filtering the returned ICMP messages, no packets
will get through.

4.  GRE uses the Linux IP stack's view of the network.  If the MTU of the
physical device is not set correctly packets can be dropped.

It's hard to tell what is going on without knowing the specifics of the
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