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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 is 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 four queues have queue weights of 1, 2, 4 and 8, packets with the highest forwarding
priority are assigned to queue 4, and packets with the lowest forwarding priority assigned to queue 1.
By providing highest forwarding priorities across 4 queues, WRR processes higher priority traffic, and
ensures that low-priority traffic is forwarded satisfactorily.
The scheduling scheme is enabled system-wide. Queues assigned to the strict priority policy are
automatically assigned to the highest priority queue. By default, all values are set as Strict Priority. Queue
weight values can be assigned in any order using WRR, and WRR values can be assigned system-wide.
Best effort traffic is always assigned to the first queue.
Table 9-2. DSCP to Queue Mapping Table Default Values
DSCP Value Forwarding Queue Values
0-15 q1
16-31 q2
32-47 q3
48-63 q4