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D-Link DES-3326SR Layer 3 Switch

Chapter 10

Quality of Service Configuration

Configure QoS Output Scheduling

Configure 802.1p User Priority

Configure Default Priority

Configure Bandwidth

The DES-3326SR switch supports 802.1p priority queuing. The switch has 4 priority queues. These priority

queues are numbered from 0 (Class 0) — the lowest priority queue — to 3 (Class 3) — the highest priority

queue. The eight priority queues specified in IEEE 802.1p (p0 to p7) are mapped to the switch’s priority queues

as follows:

p1 and p2 are assigned to the switch’s Class 0 queue.

p0 and p3 are assigned to the switch’s Class 1 queue.

p4 and p5 are assigned to the switch’s Class 2 queue.

p6 and p7 are assigned to the switch’s Class 3 queue.

Priority scheduling is implemented using two types of methods, strict priority and round-robin priority. If no

changes are made to the QoS priority scheduling settings the method used is strict priority.

For strict priority-based scheduling, packets residing in the higher priority queues are transmitted first. Only

when these queues are empty, are packets of lower priority allowed to be transmitted. Higher priority packets

always receive preference regardless of the amount of lower priority packets in the buffer and regardless of the

time elapsed since any lower priority packets have been transmitted. By default the switch is configured to

empty the buffer using strict priority.

NOTICE: The default QoS scheduling arrangement is a strict priority schedule. To

customize scheduling to set up round-robin queue clearing, the MAX. Latency and

MAX. Packets values need to be changed.

To use implement round-robin (weighted) priority, the switch’s four priority queues can be configured to reduce

the buffer in a round-robin fashion - beginning with the highest priority queue, and proceeding to the lowest

priority queue before returning to the highest priority queue.

The weighted-priority based scheduling alleviates the main disadvantage of strict priority-based scheduling − in

that lower priority queues get starved of bandwidth − by providing a minimum bandwidth to all queues for

transmission. This is accomplished by configuring the maximum number of packets allowed to be transmitted

from a given priority queue and the maximum amount of time a given priority queue will have to wait before

being allowed to transmit its accumulated packets. This establishes a Class of Service (CoS) for each of the

switch’s four hardware priority queues.

The possible range for maximum packets is: 0 to 255 packets.

The possible range for maximum latency is: 0 to 255 (in increments of 16 microseconds each).

Remember that the DES-3326SR has four priority queues (and thus four Classes of Service) for each port on the

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To configure QoS settings, open the Configure QoS subdirectory in the Advanced Setup folder, and the click

on the link for the QoS setting you want to configure. h.