CISCO IOS in a Nutshell

Name

loopback — interface

Synopsis

loopback [options]
no loopback

Configures

Loopback mode

Default

Disabled

Description

The loopback command sets the equipment at some point between a router interface and the other end of the line to reflect all data back to the router. Loopbacks are extremely useful for troubleshooting. With no options, loopback tests the local interface: all packets sent to the interface are immediately reflected back to the router without being sent to the destination. The various options and parameters allow you to place the loopback point farther down the line:

applique

Sets the internal loopback for an HSSI interface.

dte

Sets the loopback at the CSU/DSU, which tests the cable between the router (the DTE) and the CSU/DSU. The CSU/DSU must support this option.

line [payload]

Sets the loopback at the “far end” of the CSU/DSU, which sends the packets completely through the CSU/DSU and back to the router. The CSU/DSU must support this option. On routers with built-in CSU/DSUs (2524 or 2525) you can add the payload keyword, which creates the loopback at the DSU.

remote option

Sets the loopback at the remote CSU/DSU, which sends packets all the way to the remote end of the connection before reflecting them back to the router. This command tests the entire communications link between the router and the far end of the line. The remote CSU/DSU must be configured for remote loopback. Additional options give you more control over the behavior of remote loopback.

The following ...