[ovs-discuss] OVN - MTU path discovery

Ben Pfaff blp at ovn.org
Thu Nov 1 17:39:14 UTC 2018

On Tue, Oct 30, 2018 at 04:07:42PM +0530, Numan Siddique wrote:
> During the discussion, Ben proposed a new action instead which will check
> the MTU of the outport for the given port.
> something like check_mtu(router_port_xyzzy). This action would raise an
> exception if the packet size is greater
> than the MTU of the outport.  Please see [1] for the chat logs.
> I explored a bit on this new action - check_mtu. And I am not sure if we
> can solve the problem reported here using
> this action. When the chassis (hosting the gateway router port) receives
> the packet from the compute chassis
> tunnel port, it  runs the router pipeline, does NATting and sends the
> packet to the ingress pipeline of the provider
> network logical switch (with the localnet port). From the localnet patch
> port the packet will enter the provider bridge (br-ex)
> and the packet is pushed out of the physical interface. If in case mtu of
> this physical interface is lesser than the size of the
> packet, OVS drops the packet.
> The issue with the check_mtu action is that, OVN doesn't program the
> provider bridges and is unaware of the physical
> interface connected to it. So I am not sure if we can use this approach to
> solve this problem. Although there can be an
> usecase for the check_mtu action.
> Ben, do you have any comments on this ? Did I misunderstand from what you
> were trying to say ? Please correct me If my
> understanding is wrong.

I think you're right.

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