[ovs-discuss] Question on tunnel interfaces

[Jesse Gross]

On Fri, Apr 24, 2015 at 11:12 PM, Padmanabhan Krishnan <kprad1 at yahoo.com> wrote:
> Hello,
> My question is related to the sending (encap) side of the OVS tunnel bridge.
> After the tunnel bridge (say br-tun) does the encapsulation (VXLAN or GRE),
> i assume it uses the host routing table to route the packet. So, when a
> frame is received by the br-tun from an in_port, after performing the
> necessary encapsulation does OVS send it over to kernel routing module to
> forward the frame on the right interface (routing)?

That's correct.

> In my example below, the end point 173 subnet is attached to say, eth2. The
> physical port 'eth2' is _not_ connected to OVS bridge 'br-tun'. But,
> depending on the 'remote_ip', the frame gets forwarded to the right
> interface.
>
> I will be obliged, if someone can explain, in terms of flow, as how
> forwarding happens after the frame gets encapsulated (br-tun)? Does routing
> of the frame fall under the scope of OVS?
> Is it a special functionality of the 'set_tunnel' action?

OVS simply hands the packet over to the kernel IP stack. There aren't
any flows involved at this point beyond the one that got the packet to
the tunnel port. That's why the packet can be output on a port that is
not connected to OVS.