Specifications

2-3
Cisco ONS 15454 Troubleshooting and Maintenance Guide
November 2001
Chapter 2 General Troubleshooting
2.2 Network Tests
Use loopbacks and hairpins to test newly-created circuits before adding live traffic or to logically isolate
the source of a network failure. All ONS 15454 line (traffic) cards, except Ethernet cards, allow
loopbacks and hairpins.
2.2.1 Network Test Types
Facility loopbacks test the line interface unit (LIU) of a card, the EIA (electrical interface assembly),
and cabling. You put a facility loopback on a card and use a test set to run traffic over the loopback. A
successful facility loopback eliminates the LIU of the card, the EIA, or cabling plant as the cause or
potential cause of a network problem. Figure 2-1 shows a facility loopback on a DS-N card.
Figure 2-1 The facility loopback process on a DS-N card
Terminal loopbacks test a circuit path through the cross-connect card and as it loops back from the line
card being tested. Figure 2-2 shows a terminal loopback set on an OC-N card. The test set traffic comes
in on the DS-N card and goes through the cross-connect card to the OC-N card. The terminal loopback
on the OC-N card turns the signal around before it reaches the LIU and sends it through the cross-connect
card to the DS-N card. This test verifies that the cross-connect card and circuit paths are valid, but does
not test the LIU on the OC-N card. To test the LIU on an OC-N card, connect an optical test set to the
OC-N card ports and perform a facility loopback or use a loopback or hairpin on a card that is farther
along the circuit path.
Figure 2-2 The terminal loopback process on an OC-N card
Hairpin circuits bring traffic in and out on a DS-N port instead of sending the traffic onto the OC-N. A
hairpin loops back only the specific STS or VT circuit and does not cause an entire OC-N port to loop
back, which would drop all traffic on the OC-N port. The hairpin allows you to test a circuit on nodes
running live traffic.
DS1-N OC-N
XC
DS-N
DS-N
Facility loopback
Test Set A
55261
DS-N
DS-N OC-N
XC
DS-N
Test Set A
Terminal loopback
55266