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Cisco MGX 8850 (PXM1E/PXM45), Cisco MGX 8950, Cisco MGX 8830, and Cisco MGX 8880 Configuration Guide
Release 5.0.10, OL-3845-01 Rev. B0, August 16, 2004
Chapter 9 Switch Operating Procedures
Configuring a Line Loopback
Configuring a Line Loopback
If a connection fails and you do not know which end of the connection is causing the problem, putting a
line into loopback mode can help you determine what the problem is and where it occurs on a connection.
In an MGX 8830, an MGX 8850, or an MGX 8880, loopback lines provide CLI-based line level
monitoring capabilities.
When a line is put into loopback, the receiving switch takes all of the data it receives and returns it
unchanged back to the sender. The physical line in a loopback configuration is connected between a CPE
and a switch; one physical line is connected from the tx (Transmit port) of the CPE to the rx (receive)
port of a card on the switch you are testing. Another physical line is connected between the tx port of
the same card and the receive port of the CPE.
Configuring Loopback Line Tests on PXM1E and AXSM Cards
Once the physical connection is established, you need to use the CLI to put the connection into loopback
mode.
The following types of loopback are supported on PXM1E and AXSM cards:
•
Far-end line loopback - Loopback appears at the far-end of the CPE when you send a loopback
activation code from the PXM1E. The CPE enters a loop mode in which it returns the received data
back to the PXM1E. The CPE continues to return the data back until it receives a no-loopback
request.This kind of loopback can be used to run tests, such as BERT.
•
Far-end payload loopback- Loopback is similar to FarEnd loopback, except that the payload portion
of the data is re-transmitted. Framing is done by the Far end again.
•
Remote line loopback - Loopback returns the remote data back to the far end. The received data
stream is looped back into the transmit path, overriding the data stream created internally by the
framer.
•
Local loopback - Loopback allows the transmitted data to be looped back into the receiving path. It
can be used to test the internal hardware of the card.
Once your physical line is connected, you can perform a loopback test using the following procedure.
Step 1
Connect a single line to the appropriate transfer and receive ports on the backcard you want to test.
Step 2
Establish a configuration session with the active PXM1E or AXSM card using a user name with
SERVICE_GP privileges or higher.
Step 3
Enter the dsplns command to display the configuration for all lines on the current card.
Step 4
Enter the addlnloop <-line type> <bay.line> <-lpb loopback type> command.
addlnloop
-ds3 2.1 -lpb 2
Step 5
Enter the dspln -<line type> <line_num> command to verify the that the appropriate line is in the
specified loopback state.
dspln
-ds3 4.1
