Cisco ONS 15454 Troubleshooting Manual page 6

1.2 Identify Points of Failure on a DS-N Circuit Path
A hairpin circuit brings traffic in and out on a DS-N port rather than sending the traffic onto the OC-N
card. A hairpin loops back only the specific STS or VT circuit and does not cause an entire OC-N port
to loop back, thus preventing a drop of all traffic on the OC-N port. The hairpin allows you to test a
specific STS or VT circuit on nodes running live traffic.
a DS-N card.
Figure 1-7
Test Set
A cross-connect loopback tests a circuit path as it passes through the cross-connect card and loops back
to the port being tested. Testing and verifying circuit integrity often involves taking down the whole line;
however, a cross-connect loopback allows you to create a loopback on any embedded channel at
supported payloads at the STS-1 granularity and higher. For example, you can loop back a single STS-1,
STS-3c, STS-6c, etc. on an optical facility (line) without interrupting the other STS circuits.
The following restrictions apply to cross-connect loopbacks:
•
•
1.2 Identify Points of Failure on a DS-N Circuit Path
Facility (line) loopbacks, terminal (inward) 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 a DS-N circuit on a two-node, bidirectional line switched ring (BLSR).
Using a series of facility loopbacks, terminal loopbacks, and hairpins, the path of the circuit is traced
and the possible points of failure are tested and eliminated. A logical progression of five network test
procedures apply to this sample scenario:
Note The test sequence for your circuits will differ according to the type of circuit and network topology.
1.
2.
3.
4.
5.
All loopback tests require on-site personnel.
Note
Cisco ONS 15454 Troubleshooting Guide, R4.6
1-6
Hairpin Circuit Process on a DS-N Card
DS-N
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.
A facility (line) loopback on the source node DS-N
A hairpin on the source node DS-N
A terminal (inward) loopback on the destination node DS-N
A hairpin on the destination node DS-N
A facility (line) loopback on the destination DS-N
Figure 1-9
XC OC-N
Chapter 1 General Troubleshooting
shows the hairpin circuit process on
November 2004