Specifications

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Configuration for WRR and the other aspects of what have been described above are explained in the following

section.

6.5.3. Deficit Weighted Round Robin

DWRR is a feature that is used on egress (transmit) queues. Explaining DWRR is best served by using an example.

Let’s assume a switch port has 3 queues and we are using the normal WRR algorithm. Queue 1 has been given

access to 50% of the bandwidth, Queue 2 has 30% and Queue 3 has 20%. If the WRR algorithm is servicing Queue

2 and on this service pass it has used 99.9% of its allotted bandwidth, the WRR algorithm will still send out the next

packet in the queue as the queues allotted bandwidth allocation has not yet been used up. When it sends the next

packet, it will exceed the amount of bandwidth that was configured for this queue. Statically over time, Queue 2 may

end up using a lot more bandwidth than it was initially configured for when using WRR.

Figure 21. Deficit Weighted Round Robin

DWRR alleviates this problem from occurring. If the queue uses more bandwidth than it was allotted, DWRR keeps a

tally of the extra bandwidth used on that pass. On the next pass through the queues, the DWRR algorithm will

subtract the extra bandwidth used on the last pass for this turn. This means statistically over a period of time that

each queue will use bandwidth that is much closer to the configured amount for that queue.

6.5.4. Shaped Round Robin

SRR is a recent addition to the scheduling capabilities of the Catalyst 6500 family. SRR is currently available only on

egress and only on the WS-X6708, the WS-X6716, the uplink ports of the Supervisor 32 and Sup720-10GE. SRR is

different to WRR in that the SRR algorithm provides a way to shape outbound traffic to a stated rate. In some

respects, it is similar to a policer except that traffic in excess of the rate will be buffered rather than dropped as with a

policer.

SRR is configured by adding the keyword shape to the queue configuration and can only be configured if the priority

queue is not used (no COS is associated with the priority queue)

This is the CLI used to configured SRR on the uplink of the Sup720-10GE

Cat6500(config-if)#int ten 5/4

Cat6500(config-if)#wrr-queue shape 100 150 200