Configuring Serial Loopback Capability
From the router, remote line interface unit (LIU) loopback loops the TX (transmit) data and TX clock back to the router as RX (receive) data and RX clock. From the line, LIU loopback loops the RX data and RX clock back out the line as TX data and TX clock, as shown in Figure 1.
Figure 1: Serial Interface LIU Loopback
DCE local and DCE remote control the EIA-530 interface-specific signals for enabling local and remote loopback on the link partner DCE. Local loopback is shown in Figure 2.
Figure 2: Serial Interface Local Loopback
For EIA-530 interfaces, you can configure DCE local, DCE remote, local, and remote (LIU) loopback capability.
For V.35, you can configure remote LIU and local loopback capability. DCE local and DCE remote loopbacks are not supported on V.35 and X.21 interfaces. Local and remote loopbacks are not supported on X.21 interfaces.
To configure the loopback capability on a serial interface, include the loopback statement, specifying the dce-local, dce-remote, local, or remote option:
You can include this statement at the following hierarchy levels:
- [edit interfaces se-pim/0/port serial-options]
- [edit interfaces se-fpc/pic/port serial-options]
To disable the loopback capability, remove the loopback statement from the configuration:
You can determine whether there is an internal or external problem by checking the error counters in the output of the show interface se-fpc/pic/port extensive command:
Example: Configuring Serial Loopback Capability
To determine the source of a problem, loop packets on the local router, the local DCE, the remote DCE, and the remote line interface unit (LIU). To do this, include the no-keepalives and encapsulation cisco-hdlc statements at the [edit interfaces se-fpc/pic/port] hierarchy level, and the loopback local option at the [edit interfaces se-pim/0/port serial-options] or [edit interfaces se-fpc/pic/port serial-options] 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.
Published: 2012-11-27
