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Configuring E1 Loopback Capability


You can configure loopback capability between the local E1 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 E1 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 E1 interface, forwarded if there is a valid route, and immediately retransmitted to the CSU.

Figure 1: Remote and Local E1 Loopback
Remote and Local E1 Loopback

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, you 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.

To configure E1 Loopback capability on an E1 interface:

  1. In the configuration mode go to the [edit interfaces interface-name e1-options] hierarchy level.
  2. Include the loopback statement. Note that the loopback local statement causes the interface to loop within the PIC just before the data reaches the transceiver.
  3. To determine whether a problem is internal or external, loop packets on both the local and the remote router. Include the no-keepalives and encapsulation cisco-hdlc statements at the [edit interfaces interface name] hierarchy level. With this configuration, the link stays up, so you can loop ping packets to a remote router.
  4. Check the error counters in the output of the show interface interface-name extensive to determine whether there is an internal problem or an external problem.
  5. View the configuration by issuing the show command at the [edit interfaces e1-fpc/pic/port ] hierarchy level.
  • You can turn off the loopback capability by removing the loopback statement from the configuration

  • You can configure the CE1 loopback capability on the 16-port Channelized E1/T1 Circuit Emulation MIC (MIC-3D-16CHE1-T1-CE), by including the loopback statement at the [edit interfaces ce1-fpc/pic/port] hierarchy level.