System information

Troubleshooting Serial Line Problems 15-335
Implementing Hold Queue Limits
Implementing Hold Queue Limits
Hold queues are buffers used by each router interface to store outgoing or incoming packets. Use the
hold-queue interface configuration command to increase the number of data packets queued before
the router will drop packets. Increase these queues by small increments (for instance, 25%) until you
no longer see drops in the show interfaces output. The default output hold queue limit is 100
packets.
Note The hold-queue command is used for process-switched packets and periodic updates
generated by the router.
Use the hold-queue command to prevent packets from being dropped and to improve serial-link
performance under the following conditions:
• You have an application that cannot tolerate drops and the protocol is able to tolerate longer
delays. DECnet is an example of a protocol that meets both criteria. Local-area transport (LAT)
does not because it does not tolerate delays.
• The interface is very slow (bandwidth is low or anticipated utilization is likely to sporadically
exceed available bandwidth).
Note When you increase the number specified for an output hold queue, you might need to increase
the number of system buffers. The value used depends on the size of the packets associated with the
traffic anticipated for the network.
Using Priority Queuing to Reduce Bottlenecks
Priority queuing is a list-based control mechanism that allows traffic to be prioritized on an
interface-by-interface basis. Priority queuing involves two steps:
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.
Note Priority queuing automatically creates four hold queues of varying size. This overrides any
hold queue specification included in your configuration.
Use priority queuing to prevent packets from being dropped and to improve serial link performance
under the following conditions:
• When the interface is slow, there are a variety of traffic types 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.