DELL N3000 Manual De Usuario

Configuring Class-of-Service

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.

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.