Figure - Cisco ONS 15454 Troubleshooting Manual

1.1.3 Hairpin Circuits
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Caution A lock out of protection must occur before putting a two-fiber or four-fiber BLSR span into a terminal
loopback state. That is, a span lockout of one side (such as the east side) of a two-fiber BLSR is required
before operating a facility loopback on the same (east) side of the ring. A span lockout of one protection
side (such as the east protection side) of a four-fiber BLSR is required before operating a terminal
loopback on the same (east) side of the ring. If you do not perform the lockout prior to creating the
loopback, the ring can become stuck in an anomalous state after you release the loopback.
1.1.3 Hairpin Circuits
A hairpin circuit sends traffic in and out an electrical port rather than sending the traffic onto the OC-N
card. A hairpin loops back only the specific synchronous transport signal (STS) or virtual tributary (VT)
circuit and does not cause an entire OC-N port to loop back, preventing all traffic on the OC-N port from
dropping. The hairpin allows you to test a specific STS or VT circuit on nodes running live traffic.

Figure 1-9

Figure 1-9
Test Set
1.1.4 Cross-Connect Loopbacks
A cross-connect (XC) loopback tests an OC-N circuit path as it passes through the cross-connect card
and loops back to the port being tested without affecting other traffic on the optical port. Cross-connect
loopbacks are less invasive than terminal or facility loopbacks. Facility and terminal loopback testing
and circuit verification often involve taking down the whole line; however, a cross-connect loopback
allows you to create a loopback on any embedded channel at supported payloads of STS-1 granularity
and higher. For example, you can place a loopback on a single STS-6c on an optical facility without
interrupting the other STS circuits.
This test can be conducted locally or remotely through the CTC interface without on-site personnel. It
takes place only on an OC-N card and tests the traffic path on that STS (or higher) circuit through the
port and cross-connect card. The signal path is similar to a facility loopback.
The XC loopback breaks down the existing path and creates a new cross-connect—a hairpin—while the
source of the original path is set to inject a line-side AIS-P. The loopback signal path and AIS injection
are shown in
Cisco ONS 15454 Troubleshooting Guide, R8.5
1-8
When an electrical or optical port is placed in the OOS-MA,MT service state before loopback
testing, the port clears the AIS signal upstream and downstream unless there is a service-affecting
defect that also causes an AIS signal to be injected. For more information about placing ports into
alternate states for testing, refer to the "Change Card Settings" chapter of the Cisco ONS 15454
Procedure Guide.
shows the hairpin circuit path on a DS-N card.
Hairpin Circuit Path on a DS-N Card
DS-N
Figure 1-10.
XC OC-N
Chapter 1 General Troubleshooting
November 2009