Circuitless IP (CLIP) is a virtual (or loopback) interface that you do not associate with a physical port. You can use a CLIP interface to provide uninterrupted connectivity to your switch as long as an actual path exists to reach the device. For example, as shown in the following figure, a physical point-to-point link exists between R1 and R2 along with the associated addresses (195.39.1.1/30 and 195.39.1.2/30). Note also that an iBGP session exists between two additional addresses 195.39.128.1/32 (CLIP 1) and 195.39.128.2/32 (CLIP 2).
The system treats the CLIP interface like an IP interface and treats the network associated with the CLIP as a local network attached to the device. This route always exists and the circuit is always up because no physical attachment exists.
The router advertises routes to other routers in the domain either as external routes using the route-redistribution process or after you enable OSPF in a passive mode to advertise an OSPF internal route. You can configure only the OSPF protocol on the CLIP interface. After you create a CLIP interface, the system software programs a local route with the CPU as the destination ID. The CPU processes all packets destined to the CLIP interface address. The system treats other packets with destination addresses associated with this network (but not to the interface address) as if they are from an unknown host.
A circuitless IP or CLIP address is a logical IP address for network management, as well as other purposes. The CLIP is typically a host address (with a 32 bit subnet mask). Configure the OSPF router ID to the configured CLIP address. By default, the BGP router ID is automatically equivalent to the OSPF router ID.
For information about how to configure CLIP interfaces, see Configuring a CLIP Interface and Configure a Circuitless IPv4 Interface.