FAQ: Add FAQ entry for determining OpenFlow port numbers.

I don't understand why, but I've had this question from multiple people
recently.

Signed-off-by: Ben Pfaff <blp@nicira.com>

diff --git a/FAQ b/FAQ
index a6dd56825d537ebaa7df82d925b9d3ac326f127b..4ad8c4390b58dfe22eab83cdd865ce29587f1f4e 100644 (file)
--- a/FAQ
+++ b/FAQ
@@ -520,6 +520,33 @@ A: The term "normalization" in the log message means that a flow
    TCP source port 1234, write "tcp,tp_src=1234", or to match UDP
    source port 1234, write "udp,tp_src=1234".
 
+Q: How can I figure out the OpenFlow port number for a given port?
+
+A: The OFPT_FEATURES_REQUEST message requests an OpenFlow switch to
+   respond with an OFPT_FEATURES_REPLY that, among other information,
+   includes a mapping between OpenFlow port names and numbers.  From a
+   command prompt, "ovs-ofctl show br0" makes such a request and
+   prints the response for switch br0.
+
+   The Interface table in the Open vSwitch database also maps OpenFlow
+   port names to numbers.  To print the OpenFlow port number
+   associated with interface eth0, run:
+
+       ovs-vsctl get Interface eth0 ofport
+
+   You can print the entire mapping with:
+
+       ovs-vsctl -- --columns=name,ofport list Interface
+
+   but the output mixes together interfaces from all bridges in the
+   database, so it may be confusing if more than one bridge exists.
+
+   In the Open vSwitch database, ofport value -1 means that the
+   interface could not be created due to an error.  (The Open vSwitch
+   log should indicate the reason.)  ofport value [] (the empty set)
+   means that the interface hasn't been created yet.  The latter is
+   normally an intermittent condition (unless ovs-vswitchd is not
+   running).
 
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