2-4 Loopback and Null Interfaces

• Loopback interfaces are virtual in nature and are always up.

• Loopback interfaces can be used as termination points for protocols such as BGP, RSRB, and DLSW+. These interfaces are always available, even if other physical interfaces are down.

• Loopback interfaces can also be used to provide a known and stable ID for OSPF routers. OSPF chooses its ID from the loopback interface with the highest IP address first, if present. Otherwise, the ID is taken from the physical interface that has the highest IP address.

• Null interfaces are also virtual and are always up.

• Loopback interfaces can also be used whenever data needs an intermediate output interface, such as for address translation.

• Null interfaces never forward or receive traffic; packets routed to a null interface are dropped.

Configuration

1. (Optional) Define a loopback interface:

    (global)  interface loopback   number  

   The loopback interface is created and enabled. The interface can then be given network addresses. Note that a loopback interface comes up in "no shutdown" mode as soon as it is created, unlike other types of interfaces. You can still manually shut down a loopback interface if necessary.

2. (Optional) Specify the null interface:

    (global)  interface null 0  

   The null interface is selected and can be configured. There is only one null interface on a router: null 0. Usually, the null interface is referenced in other commands, such as the gateway in a static route.

Example

Two loopback interfaces are configured with IP addresses. The loopbacks will be referenced by other IOS features, and each must be assigned to a unique IP network (as with all interfaces).

  interface loopback 0   ip address 10.1.1.1 255.255.0.0   interface loopback 1   ip address 192.168.17.1 255.255.255.0