After you define all relevant VRFs on the PE-router, you must tell the PE-router which interfaces belong to which VRF and, therefore, should populate the VRF with routes from connected sites. More than one interface can belong to the same VRF.
You can do this by using the ip vrf forwarding interface-mode command, which associates the interface with the named VRF. Both main and sub-interfaces can be defined within a VRF. Example 9-13 shows the relevant configuration for the SuperCom San Jose PE-router.
hostname San Jose ! interface serial0 description ** interface to Eurobank San Francisco** ip vrf forwarding EuroBank ip address 10.2.1.5 255.255.255.252 ! interface serial1 description ** interface to FastFoods San Jose** ip vrf forwarding FastFoods ip address 22.214.171.124 255.255.255.252
When the interface is associated with a particular VRF, its IP address is removed from the global routing table and from the interface. This is because an assumption is made that the address is not valid across multiple routing tables and should be reconfigured after the interface is given membership to a VRF.
Only interfaces that run CEF switching can be associated with VRFs because the CEF switching mechanism is a necessary prerequisite for successful MPLS/VPN data forwarding as label imposition is achieved through the CEF switching path.