The external interface of a Neutron router is referred to as the gateway interface. A router is limited to a single gateway interface. To be eligible for use as an external network that can be used for router gateway interfaces, a provider network must have its router's external attribute set to True or External.

To attach a gateway interface to a router, use the openstack router set command shown here:

openstack router set --external-gateway <network> <router>

The <network>&amp;nbsp;argument represents a network name or ID to be attached to the router as the gateway network. Neutron will assign an IP address from the external network to the router. By default, Neutron routers perform source network address translation, or SNAT, on all traffic from instances that lack floating IPs. NAT and SNAT will be covered in more detail later in this chapter. To disable this functionality, use the --disable-snat&amp;nbsp;argument when using the openstack router set&amp;nbsp;command.