Dell PowerConnect 2708 Appendix page 11

Configuring Class of Service (CoS)
The underlying mechanism of Class of Service (CoS) is the concept of queues. After packets are
assigned to a specific egress queue, CoS services can be assigned to the queue(s). The
PowerConnect 2708/2716/2724 system supports four queues per port for service priorities, where
queue 0 is the lowest priority and queue 3 is the highest priority. For each logical group of priorities
defined by the user, two priority levels are defined.
Egress queues are configured with a scheduling scheme by one of the following methods:
Strict Priority — Ensures that time-sensitive applications are always forwarded. Strict Priority (SP)
allows the prioritization of mission-critical, time-sensitive traffic over less time-sensitive
applications. For example, under Strict Priority, voice over IP traffic can be prioritized so the IP
traffic is forwarded before FTP or e-mail traffic.
Weighted Round Robin — Ensures that a single application does not dominate the device
forwarding capacity. Weighted Round Robin (WRR) forwards entire queues in a Round Robin
order. All queues can participate in WRR, with expect SP queues. SP queues are serviced before
WRR queues. If the traffic flow is minimal, and SP queues do not occupy the whole bandwidth
allocated to a port, the WRR queues can share the bandwidth with the SP queues.
Ensuring the remaining bandwidth is distributed according to the weight ratio. If WRR is selected,
the following weights are assigned to the queues: 1, 2, 4, 8.
The following figure illustrates the concept of CoS priority queues managed by the device. In this
scenario, priorities are assigned to untagged incoming packets, per port.
Figure 9-11. CoS Priority Queues
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