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Configuring Class-of-Service 1315

How Are Traffic Queues Defined?

For each queue, you can specify:

• Minimum bandwidth guarantee—A percentage of the port’s maximum

negotiated bandwidth reserved for the queue. Unreserved bandwidth can

be utilized by lower-priority queues. If the sum of the minimum

bandwidth is 100%, then there is no unreserved bandwidth and no sharing

of bandwidth is possible.

• Scheduler type—strict/weighted:

– Strict priority scheduling gives an absolute priority based on CoS

queue number, with traffic in the highest numbered queue sent first,

then the next lowest numbered queue, and so on. Weighted queues

are serviced after all strict priority queues have been serviced.

– Weighted scheduling selects packets for transmission with a fixed

weighting equal to the CoS queue number plus one. The weighted

scheduler measures bandwidth based upon bytes vs. packet counts,

offering a better granularity of scheduling. For example, if CoS queues

0, 1, and 2 have an equal offered load toward a congested output port,

CoS queue 2 will receive 3/6 of the bandwidth, CoS queue 1 will

receive 2/6 of the bandwidth, and CoS queue 0 will receive 1/6 of the

bandwidth.

The minimum bandwidth setting can be used to override the strict priority

and weighted settings. The highest numbered strict priority queue will

receive no more bandwidth than 100 percent minus the sum of the minimum

bandwidths percentages assigned to the other queues. If used, it is

recommended that minimum bandwidth percentages only be high enough to

ensure a minimum level of service for any queue; i.e., the sum of the

minimum bandwidth percentages is a small fraction of 100%. This ensures

that the system can respond to bursts in traffic. Setting the minimum

bandwidth percentages such that they sum to 100% effectively sets the

scheduler such that sharing of bandwidth is disabled.

Which Queue Management Methods Are Supported?

The switch supports the following methods, configurable per-interface-

queue, for determining which packets are dropped when the queue is full:

• Taildrop—Any packet forwarded to a full queue is dropped regardless of its

priority.

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