loopback (T1 interface)
To loop individual T1 channels on the CT3IP in Cisco 7000 series routers that have the RSP7000 and RSP7000CI and in Cisco 7500 series routers, use the loopback command in interface configuration mode. To remove the loopback, use the no form of this command.
loopback [local | network {line | payload} | remote {line {fdl {ansi | bellcore} | inband} | payload [fdl] [ansi]}]
no loopback
Syntax Description
local |
(Optional) Loops the router output data back toward the router at the T1 framer and sends an alarm indication signal (AIS) signal out toward the network. |
network line | payload |
(Optional) Loops the data back toward the network before the T1 framer and automatically sets a local loopback at the High-Level Data Link Control (HDLC) controllers (line), or loops the payload data back toward the network at the T1 framer and automatically sets a local loopback at the HDLC controllers (payload). |
remote line fdl {ansi |bellcore |
(Optional) Sends a repeating, 16-bit Extended Superframe (ESF) data link code word (00001110 11111111 for FDL ANSI and 00010010 11111111 for FDL Bellcore) to the remote end requesting that it enter into a network line loopback. Specify the ansi keyword to enable the remote line Facility Data Link (FDL) ANSI bit loopback on the T1 channel, per the ANSI T1.403 specification. Specify the bellcore keyword to enable the remote SmartJack loopback on the T1 channel, per the TR-TSY-000312 specification. |
remote line inband |
(Optional) Sends a repeating, 5-bit inband pattern (00001) to the remote end requesting that it enter into a network line loopback. |
remote payload [fdl ] [ansi ] |
(Optional) Sends a repeating, 16-bit ESF data link code word (00010100 11111111) to the remote end requesting that it enter into a network payload loopback. Enables the remote payload FDL ANSI bit loopback on the T1 channel. You can optionally specify fdl and ansi , but it is not necessary. |
Command Default
No loopback is configured.
Command Modes
Interface configuration
Command History
Release |
Modification |
---|---|
11.1 CA |
This command was introduced. |
12.2(33)SRA |
This command was integrated into Cisco IOS Release 12.2(33)SRA. |
12.2SX |
This command is supported in the Cisco IOS Release 12.2SX train. Support in a specific 12.2SX release of this train depends on your feature set, platform, and platform hardware. |
Usage Guidelines
Use this command for troubleshooting purposes.
To better diagnose T1 provisioning problems, you can place the remote CSU or remote SmartJack into loopback. The loopback remotelinefdl interface configuration command allows you to place either the CSU or the SmartJack into loopback:
-
ansi --Places the CSU into loopback, per the ANSI T1.403 Specification.
-
bellcore --Places the SmartJack into loopback, per the TR-TSY-000312 Specification.
When both are configured, transmission of loss of frame (LOF) indication (yellow alarm) takes priority over transmission of some facilities data link (FDL) messages.
If the remote loopback appears not to be working, use the show controllers t3 command to determine if the given T1 is currently attempting to transmit a LOF indication (yellow alarm):
Router#
show controllers t3 0/0/0:2
T3 0/0/0 is up.
CT3 H/W Version: 5, CT3 ROM Version: 1.2, CT3 F/W Version: 2.5.9
Mx H/W version: 2, Mx ucode ver: 1.34
T1 2 is down, speed: 1536 kbs, non-inverted data
timeslots: 1-24
FDL per AT&T 54016 spec.
Transmitter is sending LOF Indication.
Receiver is getting AIS.
If the transmitter is sending a LOF indication, as in the previous example, stop the transmission of the LOF indication (yellow alarm) with the no t1 yellow generation configuration command as shown in the following example:
Router(config)# controllers t3 0/0/0
Router(config-controller)# no t1 2 yellow generation
Router(config-controller)# Ctrl-D
To verify that the transmission of the LOF indication (yellow alarm) has stopped, use the show controllers t3 command:
Router# show controllers t3 0/0/0:2
T3 0/0/0 is up.
CT3 H/W Version: 5, CT3 ROM Version: 1.2, CT3 F/W Version: 2.5.9
Mx H/W version: 2, Mx ucode ver: 1.34
T1 2 is down, speed: 1536 kbs, non-inverted data
timeslots: 1-24
FDL per AT&T 54016 spec.
Receiver is getting AIS.
Framing is ESF, Line Code is B8ZS, Clock Source is Internal.
Yellow Alarm Generation is disabled
Then retry the remote loopback command. When diagnosis is complete, remember to reenable the LOF indication (yellow alarm).
You can also loopback all the T1 channels by using the loopback (CT3IP) interface configuration command.
Examples
The following example configures T1 channel 5 for a local loopback:
Router(config)# interface serial 3/0/0:5
Router(config-if)# loopback local