Configuring Shared IP Interfaces

To share IP interfaces:

1. Create a layer 2 interface.

   host1(config)#interface atm 5/3

   host1(config-if)#interface atm 5/3.101

2. (Optional) Create a primary IP interface.

   host1(config-if)#ip address 10.1.1.1 255.255.255.255

   host1(config-if)#exit

3. Create the shared IP interface.

   host1(config)#interface ip si0

4. Associate the shared IP interface with the layer 2 interface by one of the following methods:
   • Statically

     host1(config-if)#ip share-interface atm 5/3.101

   • Dynamically

     host1:vr-a:vrf-1(config-if)#ip share-nexthop 10.0.0.1

5. To fully configure the shared interface, assign an address (or make the interface unnumbered).

   host1(config-if)#ip address 2.2.2.2 255.0.0.0

interface ip

• Use to create an IP interface for interface sharing.
• Use the specified name to refer to the shared IP interface; you cannot use the layer 2 interface to refer to them, because the shared interface can be moved.
• Example

  host1(config)#interface ip si0

• Use the no version to delete the IP interface.
• See interface ip

ip share-interface

• Use to specify the layer 2 interface used by a shared IP interface. The command fails if the layer 2 interface does not yet exist. The command is not supported (that is, it fails) if you use an RSVP tunnel (for example, tunnel mpls:1) to identify the layer 2 interface.
• If you issue this command on a shared IP interface, you cannot issue the ip share-nexthop command for the interface.
• After creating the shared IP interface, you can configure it as you do any other IP interface.
• The shared interface is operationally up when the layer 2 interface is operationally up.
• You can create operational shared IP interfaces in the absence of a primary IP interface.
• Example

  host1(config-if)#ip share-interface atm 5/3.101

• Use the no version to remove the association between the layer 2 interface and the shared IP interface. You can delete shared and primary IP interfaces independently.
• See ip share-interface

ip share-nexthop

• Use to specify that the shared IP interface dynamically tracks a next hop. If the next hop changes, the shared IP interface moves to the new layer 2 interface associated with the IP interface toward the new next hop.
• If you issue this command on a shared IP interface, you cannot issue the ip share-interface command for the interface.
• If you issue this command on a shared IP interface, the shared interface cannot dynamically track the next hop for the specified destination if the next-hop IP address is resolvable over MPLS.
• If you specify a virtual router, the command fails if the VR does not already exist. If you do not specify a VR, the current VR is assumed.
• After creating the shared IP interface, you can configure it as you do any other IP interface.
• The shared interface is operationally up when the layer 2 interface associated with the specified next hop is operationally up. However, if the layer 2 interfaced associated with the specified next hop is an MPLS next hop (for example, an RSVP or LDP tunnel), the shared interface remains operationally down.
• Use the no version to halt tracking of the next hop.
• See ip share-nexthop

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