[ovs-discuss] Frames are dropped if its input port and output port are the same

Hans Yu chy019 at eng.ucsd.edu
Sat Oct 31 20:34:44 UTC 2015

Hi Justin,

Thanks for the reply. Now I configured my controller to set output port
as OFPP_IN_PORT to take care of these frames and it worked.



On Thu, Oct 22, 2015 at 4:15 PM, Justin Pettit <jpettit at nicira.com> wrote:

> > On Oct 22, 2015, at 4:07 PM, Hans Yu <chy019 at eng.ucsd.edu> wrote:
> >
> > Hi,
> >
> > I noticed that my Ethernet frames are dropped when their input ports are
> the same as their output ports. Even though I have the right rule installed
> on to Open vSwitch bridge from my controller, they still get dropped.
> From the FAQ:
> ### 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[]
> --Justin
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