doc: Add 'faq' about datapath-id
This patch populate faq about datapath-id.
@@ -281,3 +281,28 @@ the sense of OFPP_LOCAL)
A: Open vSwitch does not support such a configuration. Bridges always have
their local ports.
+
+Q: Why does OVS pick its default datapath ID the way it does?
+
+ A: The default OpenFlow datapath ID for a bridge is the minimum
+ non-local MAC address among all of the ports in a bridge. This
+ means that a bridge with a given set of physical ports will always
+ have the same datapath ID. This is useful for virtualization
+ systems, which typically put a single physical port (or a single
+ bond of multiple ports) on a given bridge alongside the virtual
+ ports for running VMs. In such a setup, the IP address for the
+ NIC associated with a physical port gets migrated from the
+ physical NIC to the bridge port. The bridge port should have the
+ same MAC address as the physical NIC, so that the host doesn't
+ suddenly start using a different MAC, and taking the minimum MAC
+ address does this automatically and, if there is bond,
+ consistently. Virtual ports for running VMs do not affect the
+ situation because these normally have the "local" bit set, which
+ OVS ignores.
+
+ If you want a stable MAC and datapath ID, you could set your own
+ MAC by ``hwaddr`` in ``other_config`` of bridge.
+
+ ::
+
+ ovs-vsctl set bridge br-int other_config:hwaddr=3a:4d:a7:05:2a:45
