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Excess Bandwidth Sharing Proportional Rates

To determine a good excess bandwidth-sharing proportional rate to configure, choose the largest CIR (guaranteed rate) among all the logical interfaces (units). If the logical units have PIRs (shaping rates) only, then choose the largest PIR rate. However, this is not ideal if a single logical interface has a large WRR rate. This method can skew the distribution of traffic across the queues of the other logical interfaces. To avoid this issue, set the excess bandwidth-sharing proportional rate to a lower value on the logical interfaces where the WRR rates are concentrated. This improves the bandwidth sharing accuracy among the queues on the same logical interface. However, the excess bandwidth sharing for the logical interface with the larger WRR rate is no longer proportional.

As an example, consider five logical interfaces on the same physical port, each with four queues, all with only PIRs configured and no CIRs. The WRR rate is the same as the PIR for the logical interface. The excess bandwidth is shared proportionally with a rate of 40 Mbps. The traffic control profiles for the logical interfaces are shown in Table 1.

Table 1: Example Shaping Rates and WFQ Weights

Shaping Rate

Configured Queue Transmit Rate

WFQ Weight

Total Weights

(Unit 0) 10 Mbps

(95, 0, 0, 5)

(60, 0, 0, 3)

63

(Unit 1) 20 Mbps

(25, 25, 25, 25)

(32, 32, 32, 32)

128

(Unit 2) 40 Mbps

(40, 30, 20, 10)

(102, 77, 51, 26)

255

(Unit 3) 200 Mbps

(70, 10, 10, 10)

(179, 26, 26, 26)

255

(Unit 4) 2 Mbps

(25, 25, 25, 25)

(5, 5, 5, 5)

20

Even though the maximum transmit rate for the queue on logical interface unit 3 is 200 Mbps, the excess bandwidth-sharing proportional rate is kept at a much lower value. Within a logical interface, this method provides a more accurate distribution of weights across queues. However, the excess bandwidth is now shared equally between unit 2 and unit 3 (total weights = 255).