This document reviews loopback commands on Packet Over SONET (POS)
interfaces on Cisco routers, such as the Cisco 7500 Series and the Cisco 12000
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
See also Understanding
Loopback Modes on Cisco Routers.
There are no specific prerequisites for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, see the
Cisco Technical Tips
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:
Place the interface in loop internal mode, as shown below:
Router(config)# interface pos 3/0
Router(config-if)# loop internal
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.
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.
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
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
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.
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
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:
Put the remote POS interface into loopback line with the command
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."
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.