Setting Up a Loopback Interface

One of the best-kept secrets at Cisco is the use of the Loopback0 interface on a router. The Loopback interface does not apply to Cisco Catalyst switches. The sc0 port on the Catalyst switch acts like a Loopback interface because it is the only IP address assigned to the whole switch. The configuring of the Loopback interface on a router is the recommended use in many IOS features, such as RSRB, DLSW, logging, NTP, and SNMP.

The Loopback interface is a virtual interface on the router that acts as a logical system interface that is, in essence, assigned to the router as one node instead of a multiple interface router. It stays up as long as the router is operational. It has no relationship to any physical interfaces on the router, yet acts as its own logical subnet. The Loopback interface is useful for creating a common source IP address for SNMP trap messages, syslog messages, and as the primary IP address in name resolution or DNS.

Probably the only drawback of using a Loopback interface is that it requires its own IP subnet. But you can use a host mask (255.255.255.255 or /32) for this interface because no other device uses that subnet. If you have available IP address space or can allocate a range of IP subnets for the Loopback interface, it will be easier for you to manage your network, utilizing some of the configuration techniques described in this chapter.

To configure a Loopback interface on the router, use the following CLI configuration commands:

interface loopback0
ip address 10.1.2.3 255.255.255.255

If you are short on IP addresses, the Loopback interface is probably not a viable option. In that case, consider “key” physical interfaces, such as backbone-oriented interfaces, for use as source IP addresses for SNMP traps or syslog messages because those interfaces should be up most of the time.