System information

Special Serial Line Tests
Book Title
15-336
In general, start with the default number of queues when implementing priority queues. After
enabling priority queuing, monitor output drops with the show interfaces serial exec command. If
you notice that output drops are occurring in the traffic queue you have specified to be high priority,
increase the number of packets that can be queued (using the queue-limit keyword option of the
priority-list global configuration command). The default queue-limit arguments are 20 packets for
the high-priority queue, 40 for medium, 60 for normal, and 80 for low.
Note When bridging Digital Equipment Corporation (Digital) LAT traffic, the router must drop
very few packets, or LAT sessions can terminate unexpectedly. A high-priority queue depth of
about 100 (specified with the queue-limit keyword) is a typical working value when your router is
dropping output packets and the serial lines are subjected to about 50% bandwidth utilization. If the
router is dropping packets and is at 100% utilization, you need another line.
Note Another tool to relieve congestion when bridging Digital LAT is LAT compression. You can
implement LAT compression with the interface configuration command bridge-group group
lat-compression.
Special Serial Line Tests
In addition to the basic diagnostic capabilities available on routers, a variety of supplemental tools
and techniques can be used to determine the conditions of cables, switching equipment, modems,
hosts, and remote internetworking hardware. For more information, consult the documentation for
your CSU, DSU, serial analyzer, or other equipment.
CSU and DSU Loopback Tests
If the output of the show interfaces serial exec command indicates that the serial line is up but the
line protocol is down, use the CSU/DSU loopback tests to determine the source of the problem.
Perform the local loop test first, and then the remote test. Figure 15-9 illustrates the basic topology
of the CSU/DSU local and remote loopback tests.
Figure 15-9 CSU/DSU Local and Remote Loopback Tests
Note These tests are generic in nature and assume attachment of the internetworking system to a
CSU or DSU. However, the tests are essentially the same for attachment to a multiplexer with
built-in CSU/DSU functionality. Because there is no concept of a loopback in X.25 or Frame Relay
packet-switched network (PSN) environments, loopback tests do not apply to X.25 and Frame Relay
networks.
Local loop to
Router A
Remote loop to
Router B
Remote loop
to Router A
Local loop
to Router B
Router A DSU/CSU Router BDSU/CSU
PSN