Excess Bandwidth Basic Example

This basic example illustrates the interaction of the guaranteed rate, the shaping rate, and the excess rate applied to four queues. The same concepts extend to logical interfaces (units) and cases in which the user does not configure an explicit value for these parameters (in that case, the system uses implicit parameters).

In this section, the term “not applicable” (NA) means that the feature is not explicitly configured. All traffic rates are in megabits per second (Mbps).

The hardware parameters derived from the configured rates are relatively straightforward except for the excess weight. The excess rate is translated into an absolute value called the excess weight. The scheduler for an interface picks a logical unit first, and then a queue within the logical unit for transmission. Logical interfaces and queues that are within their guaranteed rates are picked first, followed by those in the excess region. If the transmission rate for a logical interface or queue is more than the shaping rate, the scheduler skips the logical interface or queue. Scheduling in the guaranteed region uses straight round-robin, while scheduling in the excess region uses weighed round-robin (WRR) based on the excess weights. The excess weights are in the range from 1 to 127, but are transparent to the user and subject to change with implementation. The weights used in this example are for illustration only.

This example uses a logical interface with a transmit rate (CIR) of 10 Mbps, a shaping rate (PIR) of 10 Mbps. The user has also configured values of transmit rate (CIR), shaping rate (PIR), and excess rate as shown in Table 70.

Table 70: Basic Example of Excess Bandwidth

The values used by the hardware based on these parameters are shown in Table 71.

Table 71: Hardware Use of Basic Example Parameters

There are a number of important points regarding excess bandwidth calculations:

• The guaranteed rates should add up to less than the logical interface guaranteed rate (10 Mbps).
• Shaping rates (PIRs) can be oversubscribed.
• Excess rates can be oversubscribed. This rate is only a ratio at which the sharing occurs.
• Each queue receives the minimum of the guaranteed bandwidth because each queue is transmitting at its full burst if it can.
• The excess (remaining) bandwidth is shared among the queues in the ratio of their excess rates. In this case, the excess bandwidth is the logical interface bandwidth minus the sum of the queue transmit rates, or 10 Mbps – 6 Mbps = 4 Mbps.
• However, transmission rates are capped at the shaping rate (PIR) of the queue. For example, Queue 0 gets 0.5 Mbps.
• Queue 0 also gets a guaranteed transmit rate (CIR) of 0.5 Mbps and is eligible for excess bandwidth calculated as 4 Mbps (10 Mbps – 6 Mbps) multipled by 10/127. However, because the shaping rate (PIR) for Queue 0 is 0.5 Mbps, the expected traffic rate is capped at 0.5 Mbps.
• Queue 1 gets its guaranteed transmit rate (CIR) of 3 Mbps. Because Queue 0 has already been dealt with, Queue 1 is eligible for sharing the excess bandwidth along with Queue 2 and Queue 3. So Queue 1 is entitled to an excess bandwidth of 4 Mbps multiplied by 50 / (30 + 30 + 50), or 1.81 Mbps.
• In the same way, Queue 2 is eligible for its guaranteed transmit rate (CIR) of 1 Mbps and an excess bandwidth of 4 Mbps multiplied by 30 / (30 + 30 + 50), or 1.09 Mbps. However, because Queue 2 has a shaping rate (PIR) of 1.5 Mbps, the bandwidth of Queue 2 is capped at 1.5 Mbps. The additional 0.59 Mbps can be shared by Queue 1 and Queue 3.
• Queue 3 is eligible for an excess of 4 Mbps multiplied by 30 / (30 + 30 + 50), or 1.09 Mbps. This total of 2.59 Mbps is still below the shaping rate (PIR) for Queue 3 (3.5 Mbps).
• The remaining bandwidth of 0.59 Mbps (which Queue 2 could not use) is shared between Queue 1 and Queue 3 in the ratio 50/30. So Queue 3 can get 0.59 multiplied by 30 / (50 + 30), or 0.22 Mbps. This gives a total of 2.81 Mbps.
• Therefore, Queue 1 gets 3 Mbps + 1.82 Mbps + (0.59 Mbps * 50 / (50 + 30)) , or approximately 5.19 Mbps.

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