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Configuring T1 Loopback Capability


You can configure loopback capability between the local T1 interface and the remote channel service unit (CSU), as shown in Figure 1. You can configure the loopback to be local or remote. With local loopback, the T1 interface can transmit packets to the CSU, but receives its own transmission back again and ignores data from the CSU. With remote loopback, packets sent from the CSU are received by the T1 interface, forwarded if there is a valid route, and immediately retransmitted to the CSU.

Figure 1: Remote and Local T1 Loopback
Remote and Local T1 Loopback

To configure loopback capability on a T1 interface, include the loopback statement at the [edit interfaces interface-name t1-options] hierarchy level:

Packets can be looped on either the local router or the remote CSU. Local and remote loopback loop back both data and clocking information.

To exchange BERT patterns between a local router and a remote router, include the loopback remote statement in the interface configuration at the remote end of the link. From the local router, issue the test interface command.

For more information about configuring BERT, see Configuring Interface Diagnostics Tools to Test the Physical Layer Connections. For more information about using operational mode commands to test interfaces, see the CLI Explorer.

For channelized T3, T1, and NxDS0 intelligent queuing (IQ) interfaces only, you can include the loopback payload statement in the configuration to loop back data only (without clocking information) on the remote router’s PIC. In payload loopback, overhead is recalculated. For T3 IQ interfaces, you can include the loopback payload statement at the [edit interfaces ct3-fpc/pic/port] and [edit interfaces t3-fpc/pic/port:channel] hierarchy levels. For T1 interfaces, you can include the loopback payload statement in the configuration at the [edit interfaces t1-fpc/pic/port:channel] hierarchy level; it is ignored if included at the [edit interfaces ct1-fpc/pic/port] hierarchy level. For NxDS0 interfaces, payload and remote loopback are the same. If you configure one, the other is ignored. NxDS0 IQ interfaces do not support local loopback.

To determine whether a problem is internal or external, you can loop packets on both the local and the remote router. To do this, include the no-keepalives and encapsulation cisco-hdlc statements at the [edit interfaces interface-name] hierarchy level and the loopback local statement at the [edit interfaces interface-name t1-options] hierarchy level, as shown in the following example:


To configure the CT1 loopback capability on the 16-port Channelized E1/T1 Circuit Emulation MIC (MIC-3D-16CHE1-T1-CE), use the loopback statement at the [edit interfaces ct1-fpc/pic/port] hierarchy level.

With this configuration, the link stays up, so you can loop ping packets to a remote router. The loopback local statement causes the interface to loop within the PIC just before the data reaches the transceiver.

To turn off the loopback capability, remove the loopback statement from the configuration:

You can determine whether there is an internal problem or an external problem by checking the error counters in the output of the show interface interface-name extensive command, for example: