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Cisco ONS 15454 Troubleshooting Guide, R8.5
November 2009
Chapter 1 General Troubleshooting
1.2 Troubleshooting Electrical Circuit Paths With Loopbacks
Figure 1-10 Network Element with SONET Cross-Connect Loopback Function
Consider the following rules when creating cross-connect loopbacks:
• You can create a cross-connect loopback on all working or protect optical ports unless the protect
port is used in a 1+1 protection group and is in working mode.
• If a terminal or facility loopback exists on a port, you cannot use the cross-connect loopback.
1.2 Troubleshooting Electrical Circuit Paths With Loopbacks
Facility loopbacks, terminal loopbacks, and hairpin circuits are often used to test a circuit path through
the network or to logically isolate a fault. Performing a loopback test at each point along the circuit path
systematically isolates possible points of failure.
The example in this section tests an electrical circuit on a two-node BLSR. Using a series of facility
loopbacks, terminal loopbacks, hairpins, and (where appropriate) cross-connect loopbacks on optical
paths carrying electrical circuits, the path of the circuit is traced and the possible points of failure are
tested and eliminated. A logical progression of eight network test procedures apply to this sample
scenario:
Note These procedures apply to DS-1, DS-3, and EC-1 cards. The test sequence for your circuits will differ
according to the type of circuit and network topology.
West-to-east direction (left to right):
1. A facility loopback on the source-node electrical port (DS-N or EC-N)
2. A hairpin on the source-node electrical port
3. An XC loopback on the destination-node OC-N STS (carrying the electrical circuit)
4. A terminal loopback on the destination-node electrical port
East-to-west direction (right to left):
1. A facility loopback on the destination-node electrical port
2. A hairpin on the destination-node electrical port
3. An XC loopback on the source-node OC-N STS (carrying the electrical circuit)
4. A terminal loopback on the source-node electrical port
124542
External signals from
and to other equipment
Internal signals to and from
other equipment in the NE
OOS
O/E
E/O
R
AIS
T
Equipment to perform
framing, scrambling, etc.
(such as signal terminating equipment)
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