Synchronous Optical NETwork (SONET)

Understanding Loopbacks on POS Links

Document ID: 18928

Updated: Dec 02, 2005



This document reviews loopback commands on Packet Over SONET (POS) interfaces on Cisco routers, such as the Cisco 7500 Series and the Cisco 12000 Series.

Loopback tests are particularly useful when the output of the show interfaces pos command indicates that the serial line is up but the line protocol is down. Perform the local loop test first using the loopback internal command, and then perform a remote test using the loopback line command.

See also Understanding Loopback Modes on Cisco Routers.



There are no specific prerequisites for this document.

Components Used

This document is not restricted to specific software and hardware versions.


For more information on document conventions, see the Cisco Technical Tips Conventions.

The loop internal Command

Issuing the interface-level command loop internal configures the POS interface to take all locally generated transmit data and return it to the receive data path. The outgoing frames are transmitted using the currently configured clocking scheme, which can be internal or the default loop time. When set to loop internal, no externally received frames are passed to internal circuitry on the POS line card. In addition, this command causes the interface to reset and the internal linecard circuitry to reinitialize. During this time, the far-end POS interface may report a brief burst of cyclic redundancy check (CRC) errors.

Listed below is a general procedure for performing a local loopback test with the loopback internal command:

  1. Place the interface in loop internal mode, as shown below:

    Router(config)# interface pos 3/0 
    Router(config-if)# loop internal
  2. Use the show interfaces pos command to determine if the line status changes from "line protocol is down" to "line protocol is up (looped)," or if it remains down.

  3. If the line protocol comes up when the interface is in local loopback mode, this suggests that the problem is occurring on the remote end of the connection or somewhere along the path.

  4. If the status line does not change state, there is a possible problem in the router or connecting cable. If the line protocol comes up, use the debug serial interface command to isolate the problem to the local interface. The values for mineseen and yourseen in the keepalives should increment every ten seconds. This information appears in the debug serial interface output. If the keepalives do not increment, there may be a problem on the interface. Swap faulty equipment as necessary.

    Note: You will need to change the encapsulation from Point to Point Protocol (PPP) to High-Level Data Link Control (HDLC), when using loopbacks. The line protocol on an interface configured with PPP comes up only when all Link Control Protocol (LCP) and Network Control Protocol (NCP) sessions are negotiated successfully.

The loopback line Command

Issuing the interface-level command loopback line configures the POS interface to take externally-received frames and apply these frames as the transmit data via the "looper". Regular transmit data originating in the POS line card is not transmitted -- only the looped receive data. All externally received data, in addition to being looped as transmit data, is passed to internal structures.

The loopback line command works with either loop timed or internal clock settings.

General Guidelines on Loopbacks

By default, the transmit clocking (frequency and phase) is derived from the received frame clocking with clock recovery circuitry. This default is known as loop timed. When connecting POS interfaces over Synchronous Optical Network (SONET)/Synchronous Digital Hierarchy (SDH) network equipment, you must use loop timing to avoid framing slips, which result in frame loss, high bit error rates (BER), and loss of signal (LOS) alarms in severe cases.

Alternately, you can use an internal crystal clock in back-to-back configurations. The router uses a mux to select either the recovered receive clock or the internal clock.

When using interface-level loopback commands, note the following:

  • Configure loopback internal as well as clock internal when connecting to a commercial carrier network. These commands lead to physical-layer alarms at initial configuration and then continually since the internal clock is not locked to that of the carrier. Thus, it drift in and out of phase, leading to frame slips and bit errors.

  • The two loopback commands are mutually exclusive. The router uses the last configured command. Issue the no loopback command to remove all configured loopbacks. To view the active loopback mode, use the show interface pos or show run command.

  • Leave keepalives enabled when running loopback tests. These periodic messages communicate sequence information, and the reception or lack of reception of them will cause operator confusion.

If you determine that the local hardware is functioning properly but you still encounter problems when attempting to establish connections over the POS link, try using the remote loopback test to isolate the problem cause.

Note: This remote loopback test assumes that HDLC encapsulation is being used with keepalives enabled.

The following steps are required to perform loopback testing:

  1. Put the remote POS interface into loopback line with the command loopback line.

  2. Using the show interfaces pos command, determine if the line protocol remains up or if it goes down with the status line indicating "line protocol is down."

  3. If the line protocol remains up, the problem is probably at the remote end of the connection. Perform both local and remote tests at the remote end to isolate the problem source.

    If the line status changes to "line protocol is down" when switching from local to remote loopback, contact your WAN network manager or the WAN service organization since this condition suggests that a problem along the end-to-end path is preventing the return of the HDLC keepalives.

    See also Troubleshooting "Line Protocol is Down" Problems on POS Interfaces.

Related Information

Updated: Dec 02, 2005
Document ID: 18928