diff --git a/lib/neutron-legacy b/lib/neutron-legacy
index b1b5230fdd..1b7c4db507 100644
--- a/lib/neutron-legacy
+++ b/lib/neutron-legacy
@@ -211,13 +211,15 @@ PHYSICAL_NETWORK=${PHYSICAL_NETWORK:-public}
 # Example: ``OVS_PHYSICAL_BRIDGE=br-eth1``
 OVS_PHYSICAL_BRIDGE=${OVS_PHYSICAL_BRIDGE:-br-ex}
 
+default_route_dev=$(ip route | grep ^default | awk '{print $5}')
+die_if_not_set $LINENO default_route_dev "Failure retrieving default route device"
 # With the linuxbridge agent, if using VLANs for tenant networks,
 # or if using flat or VLAN provider networks, set in ``localrc`` to
 # the name of the network interface to use for the physical
 # network.
 #
 # Example: ``LB_PHYSICAL_INTERFACE=eth1``
-LB_PHYSICAL_INTERFACE=${LB_PHYSICAL_INTERFACE:-}
+LB_PHYSICAL_INTERFACE=${LB_PHYSICAL_INTERFACE:-$default_route_dev}
 
 # When Neutron tunnels are enabled it is needed to specify the
 # IP address of the end point in the local server. This IP is set