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IQE PIC Excess Bandwidth Sharing Overview

On some types of PICs, including the IQ and IQ2, and Enhanced Queuing DPCs, you can configure either a committed information rate (CIR) using the guaranteed-rate statement or a peak information rate (PIR) using the shaping-rate statement. You can configure both a PIR and CIR, and in most cases the CIR will be less than the value of PIR. For bursty traffic, the CIR represents the average rate of traffic per unit time and the PIR represents the maximum amount of traffic that can be transmitted in a given interval. In other words, the PIR (shaping-rate) establishes the maximum bandwidth available. The CIR (guaranteed-rate) establishes the minimum bandwidth available if all sources are active at the same time. Theoretically, the PIR or CIR can be established at the queue, logicial interface, or physical interface level. In this section, the PIRs or CIRs apply at the queue or logical interface (or both) levels.

Note: You can configure a shaping rate at the physical interface, logical interface, or queue level. You can configure a guaranteed rate or excess rate only at the logical interface and queue level.

Once all of the bandwidth guarantees (the sum of the CIRs at that level) are met, there could still be some excess bandwidth available for use. In existing PICs, you have no control over how this excess bandwidth is used. For example, consider the situation shown in Table 69 regarding a 10-Mbps physical interface. This example assumes that all queues are of the same priority. Also, if you do not specify a priority for the excess bandwidth, the excess priority will be the same as the normal priority.

Table 69: Default Handling of Excess Traffic

Queue	Transmit rate (CIR)	Shaping Rate (PIR)	Traffic Rate	Guaranteed Rate (Total = 6 Mbps)	Maximum Rate	Excess Bandwidth (Part of 4 Mbps Excess)	Expected Transmit Rate (Guarantee + Excess)
Q0	10%	80%	10 Mbps	1 Mbps	8 Mbps	0.73 Mbps	1.73 Mbps
Q1	20%	50%	10 Mbps	2 Mbps	5 Mbps	1.45 Mbps	3.45 Mbps
Q2	5%	5%	10 Mbps	0.5 Mbps	0.5 Mbps	0 Mbps	0.5 Mbps
Q3	25%	NA (“100%”)	10 Mbps	2.5 Mbps	10 Mbps	1.82 Mbps	4.32 Mbps

A 10-Mbps interface (the Traffic Rate column) has four queues, and the guaranteed rates are shown as percentages (Transmit Rate column) and in bits per second (Guaranteed Rate column). The table also shows the shaping rate (PIR) as a percentage (Shaping Rate column) and the actual maximum possible transmitted rate (Transmitted Rate column) on the oversubscribed interface. Note the guaranteed rates (CIRs) add up to 60% of the physical port speed or 6 Mbps. This means that there are 4 Mbps of “excess” bandwidth that can be used by the queues. This excess bandwidth is used as shown in the last two columns. One column (the Excess Bandwidth column) shows the bandwidth partitioned to each queue as a part of the 4-Mbps excess. The excess 4 Mbps bandwidth is shared in the ratio of the transmit rate (CIR) percentages of 10, 20, 5, and 25, adjusted for granularity. The last column shows the transmit rate the users can expect: the sum of the guaranteed rate plus the proportion of the excess bandwidth assigned to the queue.

Note that on PICs other than the IQE PICs the user has no control over the partitioning of the excess bandwidth. Excess bandwidth partitioning is automatic, simply assuming that the distribution and priorities of the excess bandwidth should be the same as the distribution and priorities of the other traffic. However, this might not always be the case and the user might want more control over excess bandwidth usage.

For more information on how excess bandwidth sharing is handled on the Enhanced Queuing DPC, see Configuring Excess Bandwidth Sharing.

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