The 'gateway_external_network_id' option must be configured for this agent as Neutron has more than one external network

Issue

• Multiple external network configured to use the same interface
• router connected to first external network works
• router/s connected to additional external networks show external gateway interface is down
• The following errors are in "/var/log/neutron/l3-agent.log "

ERROR neutro:n.agent.l3.agent [-] Failed to process compatible router:  696d7b53-8ec8-49f4-a44a-1ceb6ae5c0ee
ERROR neutron.agent.l3.agent Traceback (most recent call last):
ERROR neutron.agent.l3.agent   File "/usr/lib/python2.7/site-packages/neutron/agent/l3/agent.py", line 508, in _process_router_update
ERROR neutron.agent.l3.agent     self._process_router_if_compatible(router)
ERROR neutron.agent.l3.agent   File "/usr/lib/python2.7/site-packages/neutron/agent/l3/agent.py", line 434, in _process_router_if_compatible
ERROR neutron.agent.l3.agent     target_ex_net_id = self._fetch_external_net_id()
ERROR neutron.agent.l3.agent   File "/usr/lib/python2.7/site-packages/neutron/agent/l3/agent.py", line 311, in _fetch_external_net_id
ERROR neutron.agent.l3.agent     raise Exception(msg)
ERROR neutron.agent.l3.agent Exception: The 'gateway_external_network_id' option must be configured for this agent as Neutron has more than one external network.
ERROR neutron.agent.l3.agent