Specifications

2-4
Cisco ONS 15454 Troubleshooting and Maintenance Guide
November 2001
Chapter 2 General Troubleshooting
Figure 2-3 The hairpin circuit process on an OC-N card
2.2.2 Network Test Procedures
Facility loopbacks, terminal loopbacks, and hairpin circuits are often used together to test the circuit path
through the network or to logically isolate a fault. Performing a network test at each point along the
circuit path systematically eliminates possible points of failure. This example 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 eliminated.
A logical progression of five network test procedures apply to this scenario: a facility loopback on the
source node DS-N card, a hairpin on the source node DS-N card, a hairpin on the destination node OC-N
card, a terminal loopback to the destination node DS-N card, and a facility loopback to the destination
DS-N card.
Procedure: Perform a Facility Loopback on a Source DS-N Card
The first loopback test is a facility test performed on the first card in the circuit; in this example, the
DS1-14 card in source node. Completing a successful facility loopback on this card eliminates the
cabling, the DS-N card, and the EIA as possible failure points.
Figure 2-4 Facility loopback on a source DS-N card
Caution Performing a loopback on an in-service circuit is service affecting.
Note Loopbacks operate only on in-service ports.
DS-N
DS-N OC-N
XC
DS-N
Test Set A
55485
Hairpin circuit
DS-N OC-N
XC
ONS 15454
Source
OC-N DS-N
XC
ONS 15454
Destination
DS-N
DS-N
Facility loopback
Test Set A
55262