Specifications

White Paper

2 70[1] 100[2]

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Should WRED not be available on a port, the port will use a Tail Drop method of buffer management. Tail Drop, as

its name implies, simply drops incoming frames once the buffers have been fully utilized

6.5.2. WRR

WRR is used to schedule egress traffic from transmit queues. A normal round robin algorithm will alternate between

transmit queues sending an equal number of packets from each queue before moving to the next queue. The

weighted aspect of WRR allows the scheduling algorithm to inspect a weighting that has been assigned to each

queue. This defines how much bandwidth each queue has access too. The WRR scheduling algorithm will empty out

more data from queues with a higher weighting (each interval) than other queues, thus providing a bias for

designated queues.

Figure 20. Weighted Round Robin

When configuring WRR, a weight is assigned to the queue. The weight number is a number from 1 to 255. It is

important to point out that this number does not represent a percentage. If you had two queues, and you wanted

queue 1 to have 4 times as much bandwidth as queue2, then you could configure the weights as 4 and 1, 8 and 2,

80 and 20, 100 and 25, etc. As you can see, the weights do not need to add up to 100, rather they represent a ratio

that is used by the WRR algorithm to assign bandwidth to individual queues.

WRR configurable settings can be seen in the following show output:

Cat6500#show queueing int gi 5/1 det

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Transmit queues [type = 1p3q4t]:

Queue Id Scheduling Num of thresholds

-----------------------------------------

01 WRR 04

02 WRR 04

03 WRR 04

04 Priority 01

WRR bandwidth ratios: 100[queue 1] 150[queue 2] 200[queue 3]

queue-limit ratios: 50[queue 1] 20[queue 2] 15[queue 3] 15[Pri Queue]