To create an internal router interface and attach it to a subnet, use the openstack router add subnet command shown here:
openstack router add subnet <router> <subnet>
The subnet&nbsp;argument represents a subnet name or ID to be attached to the router. Neutron will assign the gateway IP of the subnet to the router when creating the qr interface and corresponding port.
To attach an existing port directly to the router, use the openstack router add port command shown here:
openstack router subnet add <router> <port>
The port keyword represents a port name or ID to be attached to the router.
A Neutron router can only have one interface in a given subnet, but can be attached to multiple subnets simultaneously. The L3 agent is responsible for connecting interfaces within the router namespace to the proper bridges on the host.
In Neutron, a network may contain multiple subnets. A router must be connected to all subnets in a network to properly route traffic for that network. Be sure not to attach an external network using this process, as traffic may be negatively impacted!