Hardware manual

loopback (interface)
Interface Commands IR-135
loopback (interface)
To diagnose equipment malfunctions between interface and device, use the loopback interface
configuration command. The no form of this command disables the test.
loopback
no loopback
Syntax Description
This command has no arguments or keywords.
Default
Disabled
Command Mode
Interface configuration
Usage Guidelines
This command first appeared in Cisco IOS Release 10.0.
On HSSI serial interface cards, the loopback function configures a two-way internal and external
loop on the HSA applique of the specific interface.
On MCI and SCI serial interface cards, the loopback functions when a CSU/DSU or equivalent
device is attached to the router or access server. The loopback command loops the packets through
the CSU/DSU to configure a CSU loop, when the device supports this feature.
On the MCI and MEC Ethernet cards, the interface receives back every packet it sends when the
loopback command is enabled. Loopback operation has the additional effect of disconnecting
network server functionality from the network.
On the CSC-FCI FDDI card, the interface receives back every packet it sends when the loopback
command is enabled. Loopback operation has the additional effect of disconnecting network server
functionality from the network.
On all Token Ring interface cards (except the 4-megabit CSC-R card), the interface receives back
every packet it sends when the loopback command is enabled. Loopback operation has the
additional effect of disconnecting network server functionality from the network.
Note Loopback does not work on an X.21 DTE because the X.21 interface definition does not
include a loopback definition.
To show interfaces currently in loopback operation, use the show interfaces loopback EXEC
command.
Example
The following example configures the loopback test on Ethernet interface 4:
interface ethernet 4
loopback