Troubleshooting guide

5-9
ATM and Layer 3 Switch Router Troubleshooting Guide
OL-1969-02
Chapter 5 Troubleshooting Switch Router ATM Interface Connections
Performing OAM Loopback Tests
OAM Loopback Testing
You can use the loopback test to pinpoint faults by looping a signal at various points in the network. Use
the loopback test before and after the initiation of service. Figure 5-2 shows how ATM OAM cell
loopbacks are performed first across the interface and then across different segments of the connection.
Figure 5-2 Loopback Testing Process
An ATM switch router generates the OAM cells and forwards them to another network element, which
is responsible for returning them to the generating network elements.
Each loopback cell contains the ID of the generating network element and the ID of the network element
that is looping the cells back to the originator. Any intermediate site must pass the cells on to the
loopback site (the farthest point to which the cells progress) and the generating site (the point to which
the cells return).
See Appendix C, “ATM Cell Structures,” for a format description of the OAM loopback cell.
The ATM switch router provides the following three types of loopback tests:
• Diagnostic
• Line
• PIF (physical interface)
Note If the loopback test is successful, data is reaching the I/O module properly. However, a successful
test does not verify whether the I/O module correctly encodes the data sent onto the line.
Local ATM
switch router
Intermediate ATM
switch router
Remote ATM
switch router
1. Generate
cells
Source ID
3. Examine
cells
2. Pass
cells
3. Loopback
cells
4. Pass
cells
2. Loopback
cells
Local ATM
switch router
Intermediate ATM
switch router
Remote ATM
switch router
1. Generate
cells
Source ID
5. Examine
cells
Loopback location ID
10305