Queuing on Catalyst Switches

Multiple queues protect the queue containing important traffic (voice) from drops.

The number of queues available depends upon the switch model and port type.

On some switches, "drop thresholds" can be assigned to each queue.

On some switches, queues can have normal tail drop or WRED dropping.

Drops happen in data-only queue(s).

In a converged network, it is vital to ensure that voice traffic is not dropped. The use of multiple queues in Catalyst switches protects the queue containing important traffic (voice) from being dropped. Cisco Catalyst switches offer a variety of queuing capabilities, depending upon the switch model and port type.

One of the key options that you can assign to queues in most Catalyst switches is "drop thresholds." You can assign one or more drop thresholds to a queue. Packets are queued until the thresholds are exceeded.

For example, all packets with differentiated services code points (DSCPs) that are assigned to the first threshold are dropped until the threshold is no longer exceeded. However, packets assigned to a second threshold continue to be queued and sent as long as the second threshold is not exceeded. The thresholds are all specified as percentages ranging from 1 to 100. A value of 10 indicates a threshold in which the buffer is 10 percent full.

On some switches, queues can have normal tail drop or weighted random early detection (WRED) dropping.

Drops will occur only in data queues. The purpose of using multiple queues is to prevent voice traffic from being dropped or delayed.

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