Defining Secondary Routing Table Lookup

You can enable secondary routing table lookup on the virtual router routing table of the parent (global) virtual router. The secondary lookup takes place when the initial route lookup on a VRF is unsuccessful. You can define secondary routing table lookup outside the context of the VRF or inside the context of the VRF.

To configure secondary routing table lookup from outside the VRF context:

1. Select the interface.

   host1:vr1(config)#interface gigabitEthernet 1/0

2. Specify a VRF and that you want it to perform secondary routing table lookup.

   host1:vr1(config-if)#ip vrf forwarding vrfA fallback global

   host1:vr1:vrfA(config-if)#ip address 10.12.4.5 255.255.255.0

To specify from inside the VRF context that an interface use the fallback global routing table lookup:

1. Select the interface.

   host1:vr1(config)#interface gigabitEthernet 1/0

2. Enter the VRF context.

   host1:vr1(config-if)#virtual-router :vrfA

3. Specify that the VRF perform a secondary routing table lookup.

   host1:vr1:vrfA(config-if)#ip fallback global

ip fallback global

• Use to specify secondary routing table lookup for an interface in a VRF if an initial routing table lookup is unsuccessful.
• Example

  host1:vr1:vrfA(config-if)#ip fallback global

• Use the no version to discontinue secondary routing table lookup.
• See ip fallback global.

ip vrf forwarding

• Use to assign a VRF to an interface or subinterface by forwarding the interface from the VR to the VRF. This command also enables you to specify secondary routing table lookup for a VRF if an initial routing table lookup is unsuccessful.
• Forwarding the interface removes the IP configuration from the interface. You must reassign an IP address to the interface after you issue this command.
• The ip vrf forwarding command changes the prompt to indicate that the CLI is now in Interface Configuration mode within the child VRF. This condition persists only for as long as you are configuring attributes on the given interface within the VRF. Entering a top-level command, such as interface, within this VRF context takes the CLI out of the VRF context back to the parent VR context.
• When you issue the ip vrf forwarding command from within the Interface Configuration or Subinterface Configuration mode of the parent VR, the IP address and other attributes of the interface are deleted from the interface. You must then reconfigure the IP attributes in the context of the VRF after issuing the command.
• Example

  host1:vr1(config-if)#ip vrf forwarding vrfA

  host1:vr1:vrfA(config-if)#ip address 10.12.4.5 255.255.255.0

  or

  host1:vr1(config-if)#ip vrf forwarding vrfA fallback global

  host1:vr1:vrfA(config-if)#ip address 10.12.4.5 255.255.255.0

• Use the no version to remove the interface assignment or discontinue secondary routing table lookup.
• See ip vrf forwarding.

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