Configuring Excess Bandwidth Sharing

Excess Bandwidth Sharing and Minimum Logical Interface Shaping

The default excess bandwidth sharing proportional rate is 32.65 Mbps (128 Kbps x 255). In order to have better weighed fair queuing (WFQ) accuracy among queues, the shaping rate configured should be larger than the excess bandwidth sharing proportional rate. Some examples are shown in Table 1.

With a 10-Mbps shaping rate, the total weights are 76. This is divided among the four queues according to the configured transmit rate. Note that when the shaping rate is larger than the excess bandwidth sharing proportional rate of 32.65 Mbps, the total weights on the logical interface are 257 and the WFQ accuracy is the same.

Selecting Excess Bandwidth Sharing Proportional Rates

A good excess bandwidth sharing proportional rate to configure is to 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 weighed round-robin (WRR) rate. This 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 2.

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 weight of each = 255).

Mapping Calculated Weights to Hardware Weights

The calculated weight in a traffic control profile is mapped to hardware weight, but the hardware only supports a limited WFQ profile. The weights are rounded to the nearest hardware weight according to the values in Table 3.

From the table, as an example, the calculated weight of 18.9 is mapped to a hardware weight of 18, because 18 is closer to 18.9 than 20 (an interval of 2 applies in the range 18–42).

Allocating Weight with Only Shaping Rates or Unshaped Logical Interfaces

Logical interfaces with only shaping rates (PIRs) or unshaped logical interfaces (units) are given a weight of 10. A logical interface with a small guaranteed rate (CIR) might get an overall weight less than 10. In order to allocate a higher share of the excess bandwidth to logical interfaces with a small guaranteed rate in comparison to the logical interfaces with only shaping rates configured, a minimum weight of 20 is given to the logical interfaces with guaranteed rates configured.

For example, consider a logical interface configuration with five units, as shown in Table 4.

The weights for these units are calculated as follows:

• Select the excess bandwidth sharing proportional rate to be the maximum CIR among all the logical interfaces: 20 Mbps (unit 2).

• Unit 1 has a PIR and unit 4 is unshaped. The weight for these units is 10.

• The weight for unit 1 queue 0 is 9.5 (10 x 95%), which translates to a hardware weight of 10.

• The weight for unit 1 queue 1 is 0 (0 x 0%), but although the weight is zero, a weight of 1 is assigned to give minimal bandwidth to queues with zero WRR.

• Unit 5 has a very small CIR (1 Mbps), and a weight of 20 is assigned to units with a small CIR.

• The weight for unit 5 queue 0 is 19 (20 x 95%), which translates to a hardware weight of 18.

• Unit 3 has a CIR of 20 Mbps, which is the same as the excess bandwidth sharing proportional rate, so it has a total weight of 255.

• The weight of unit 3 queue 0 is 127.5 (255 x 50%), which translates to a hardware weight of 128.

Sharing Bandwidth Among Logical Interfaces

As a simple example showing how bandwidth is shared among the logical interfaces, assume that all traffic is sent on queue 0. Assume also that there is a 40-Mbps load on all of the logical interfaces. Configuration details are shown in Table 5.

1. When the port is shaped at 40 Mbps, because units 2 and 3 have a guaranteed rate (CIR) configured, both units 2 and 3 get 20 Mbps of shared bandwidth.

2. When the port is shaped at 100 Mbps, because units 2 and 3 have a guaranteed rate (CIR) configured, each of them can transmit 20 Mbps. On units 1, 2, 3, and 4, the 60 Mbps of excess bandwidth is shaped according to the values shown in Table 6.

However, unit 3 only has 20 Mbps extra (PIR and CIR) configured. This means that the leftover bandwidth of 16.22 Mbps (36.22 Mbps – 20 Mbps) is shared among units 1, 2, and 4. This is shown in Table 7.

Finally, Table 8 shows the resulting allocation of bandwidth among the logical interfaces when the port is configured with a 100-Mbps shaping rate.