[ovs-discuss] Question on tunnel interfaces

Ben Pfaff blp at nicira.com
Mon Apr 27 15:38:17 UTC 2015

On Sat, Apr 25, 2015 at 06:12:54AM +0000, Padmanabhan Krishnan 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)?
> 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?

After encapsulation, OVS turns the packet over to the kernel TCP/IP
stack, which routes it appropriately.  OVS doesn't have control over how
exactly that happens, other than by controlling addressing.

The set_tunnel action just sets the tunnel key (e.g. in GRE the GRE key,
in VXLAN the VNI).

> Dumping the flow on br-tun using ofctl, i assume the below line is responsible for the encap:"
>  cookie=0x0, duration=14723.832s, table=22, n_packets=22, n_bytes=1826, idle_age=14704, dl_vlan=1 actions=strip_vlan,set_tunnel:0x3eb,output:2
> "
> Port 2 is vxlan-ad24f10f below.

I'd expect to see more information than just the tunnel key.  Check the
"ovs-dpctl show" output for port 2.

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