[ovs-discuss] input port == output port

[Ben Pfaff]

On Wed, Mar 11, 2015 at 01:53:07AM +0100, null pointer wrote:
> For a special case I need to add a flow to open vswitch that would allow me
> to set the input port as the output port for the switch.
> Currently even if I add such a flow, the packet is by default dropped,
> which is expected, in order to avoid a loop.
> Which module should I alter/change to allow that.

The FAQ says:

### Q: I added a flow to send packets out the ingress port, like this:

       ovs-ofctl add-flow br0 in_port=2,actions=2

   but OVS drops the packets instead.

A: Yes, OpenFlow requires a switch to ignore attempts to send a packet
   out its ingress port.  The rationale is that dropping these packets
   makes it harder to loop the network.  Sometimes this behavior can
   even be convenient, e.g. it is often the desired behavior in a flow
   that forwards a packet to several ports ("floods" the packet).

   Sometimes one really needs to send a packet out its ingress port
   ("hairpin"). In this case, output to OFPP_IN_PORT, which in
   ovs-ofctl syntax is expressed as just "in_port", e.g.:

       ovs-ofctl add-flow br0 in_port=2,actions=in_port

   This also works in some circumstances where the flow doesn't match
   on the input port.  For example, if you know that your switch has
   five ports numbered 2 through 6, then the following will send every
   received packet out every port, even its ingress port:

       ovs-ofctl add-flow br0 actions=2,3,4,5,6,in_port

   or, equivalently:

       ovs-ofctl add-flow br0 actions=all,in_port

   Sometimes, in complicated flow tables with multiple levels of
   "resubmit" actions, a flow needs to output to a particular port
   that may or may not be the ingress port.  It's difficult to take
   advantage of OFPP_IN_PORT in this situation.  To help, Open vSwitch
   provides, as an OpenFlow extension, the ability to modify the
   in_port field.  Whatever value is currently in the in_port field is
   the port to which outputs will be dropped, as well as the
   destination for OFPP_IN_PORT.  This means that the following will
   reliably output to port 2 or to ports 2 through 6, respectively:

       ovs-ofctl add-flow br0 in_port=2,actions=load:0->NXM_OF_IN_PORT[],2
       ovs-ofctl add-flow br0 actions=load:0->NXM_OF_IN_PORT[],2,3,4,5,6

   If the input port is important, then one may save and restore it on
   the stack:

        ovs-ofctl add-flow br0 actions=push:NXM_OF_IN_PORT[],\
                                       load:0->NXM_OF_IN_PORT[],\
                                       2,3,4,5,6,\
                                       pop:NXM_OF_IN_PORT[]