Dell PowerConnect 54 Series, User Manual

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Configuring Quality of Service
Packets arriving untagged are assigned a default VPT that is set on a per port basis. The assigned VPT is
used to map the packet to the output queue and as the egress VPT.
DSCP values can be mapped to priority queues. The following table contains the default DSCP mapping to
forwarding queue values:
DSCP mapping is enabled on a per-system basis.
CoS Services
After packets are assigned to a specific queue, CoS services can be assigned to the queue(s). Output queues are
configured with a scheduling scheme by one of the following methods:
• Strict Priority — Ensures that time-sensitive applications are always forwarded through an expedited
path. Strict Priority allows the prioritization of mission-critical, time-sensitive traffic over less time-
sensitive applications.
For example, under Strict Priority, voice over IP traffic is forwarded before FTP or e-mail (SMTP) traffic.
The strict priority queue is emptied before the traffic in the remaining queues in forwarded.
• 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. Queue priorities
are defined by the queue length. The longer the queue length, the higher the queue's forwarding priority.
For example, if eight queues have queue weights of 1, 2, 3, 4, 5, 6, 7 and 8, packets with the highest
forwarding priority are assigned to queue 8, and packets with the lowest forwarding priority assigned to
queue 1.
By providing highest forwarding priority to length 8 queues, weighted round robin processes higher
priority traffic, and ensure that low-priority traffic is forwarded satisfactorily.
Table 9-2. DSCP to Queue Mapping Table Default Values
DSCP Value Forwarding Queue Values
0-7 q1
8-15 q2
16-23 q3
24-31 q4
32-39 q5
40-47 q6
48-55 q7
56-63 q8