Hardware manual

loopback line

IR-144

Cisco IOS Interface Command Reference

loopback line

Use the loopback line interface conﬁguration command, to loop packets completely through the

CSU/DSU to conﬁgure the CSU loop. To remove the loop, use the no form of this command.

loopback line [payload]

no loopback line [payload]

Syntax Description

Default

Disabled

Command Mode

Interface conﬁguration

Usage Guidelines

This command ﬁrst appeared in Cisco IOS Release 10.0.

This command is useful for testing the DCE device (CSU/DSU) itself. When the loopback line

command is conﬁgured on the 2-wire 56-kbps CSU/DSU module or the 4-wire 56/64-kbps

CSU/DSU modules, the network data loops back at the CSU and the router data loops back at the

DSU. If the CSU/DSU is conﬁgured for switched mode, you must have an established connection to

perform a payload-line loopback. To loop the received data through the minimum amount of

CSU/DSU circuitry, issue the loopback line command.

When you issue the loopback line payload command on an integrated CSU/DSU module, the router

cannot transmit data through the serial interface for the duration of the loopback. Choosing the DSU

as a loopback point loops the received-network data through the maximum amount of CSU/DSU

circuitry. Data is not looped back to the serial interface. An active connection is required when

operating in switched mode for payload loopbacks.

If you enable the loopback line command on the fractional T1/T1 module, the CSU/DSU performs

a full-bandwidth loopback through the CSU portion of the module and data transmission through the

serial interface is interrupted for the duration of the loopback. No reframing or corrections of bipolar

violation errors or cyclic redundancy check (CRC) errors are performed. When you conﬁgure the

loopback line payload command on the FT1/T1 module, the CSU/DSU performs a loopback

through the DSU portion of the module. The loopback line payload command reframes the data

link, regenerates the signal, and corrects bipolar violations and Extended Super Frame CRC errors.

When performing a T1-line loopback with Extended Super Frame, communication over the facilities

data link is interrupted, but performance statistics are still updated. To show interfaces currently in

loopback operation, use the show service-module EXEC command.

To show interfaces currently in loopback operation on other routers, use the show interfaces

loopback EXEC command.

payload (Optional) Conﬁgures a loopback point at the DSU and loops back data to the

network on an integrated CSU/DSU.