Information Technology Reference
In-Depth Information
Step 1
Create a priority list by protocol type and level of priority.
Step 2
Assign the priority list to a specific interface.
Both of these steps use versions of the
priority-list
global configuration command. In addition, further
traffic control can be applied by referencing
access-list
global configuration commands from
priority-list
specifications. For examples of defining priority lists and for details about command syntax
associated with priority queuing, refer to the Cisco IOS configuration guides and command references.
Priority queuing automatically creates four hold queues of varying size. This overrides any
hold queue specification included in your configuration.
Note
Use priority queuing to prevent packets from being dropped and to improve serial link performance
under the following conditions:
•
When the interface is slow, a variety of traffic types are being transmitted, and you want to improve
terminal traffic performance
•
If you have a serial link that is intermittently experiencing very heavy loads (such as file transfers
occurring at specific times), and priority queuing will help select which types of traffic should be
discarded at high traffic periods
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 that 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.
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 percent
bandwidth utilization. If the router is dropping packets and is at 100 percent 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.
