Users Guide

Class-of-Service 1569
How Are Traffic Queues Configured?
The switch CoS queues may be configured to selectively service packets
queued for transmission in a pre-defined manner when an interface is
congested. If an interface is not congested, packets are transmitted in FIFO
order. The switch supports 7 queues. By default, the switch selects packets in
higher numbered queues more often than lower numbered queues while still
ensuring fairness as described below. It is recommended that administrators
utilize the lower numbered queues (0-3) to provide services as control plane
protocols are internally mapped onto the higher numbered queues (4-7).
For each queue, the following can be specified:
• Minimum bandwidth guarantee—A percentage of the port’s maximum
negotiated bandwidth reserved for the queue. Unreserved bandwidth can
be utilized by all queues, including strict priority queues. If the sum of the
minimum bandwidth is 100%, then there is no unreserved bandwidth and
no sharing of bandwidth is possible. This type of configuration will
override the strict priority configuration when congestion is present.
• 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 approximately 3/6 of the bandwidth, CoS
queue 1 will receive approximately 2/6 of the bandwidth, and CoS
queue 0 will receive approximately 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