Dell PowerConnect 2708 Appendice - Pagina 11

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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