Examples of Excess Bandwidth Sharing Calculations on the IQE PIC

The following four examples show calculations for the PIR mode. In PIR mode, the transmit rate and shaping rate calculations are based on the shaping rate of the logical interface. All calculations assume that one logical interface (unit) is configured with a shaping rate (PIR) of 10 Mbps and a scheduler map with four queues.

The first example has only a shaping rate (PIR) configured on the queues, as shown in Table 78.

Table 78: PIR Mode, with No Excess Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 79.

Table 79: PIR Mode, with No Excess Hardware Behavior

In this first example, all four queues will initially be serviced round-robin. Because there are no transmit rates configured on any of the queues, they receive a default “remainder” transmit rate of 2.5 Mbps per queue. But because there are shaping rates configured, the excess weights are calculated based on the shaping rates. For the traffic sent to each queue, Queue 0 and Queue 3 get their transmit rates of 2.5 Mbps and Queue 1 gets 1 Mbps. The remaining 4 Mbps is excess bandwidth and is divided between Queue 0 and Queue 3 in the ratio of the shaping rates (80/30). So Queue 3 expects an excess bandwidth of 4 Mbps * (30% / (80% + 30%)) = 1.09 Mbps However, because the shaping rate on Queue 3 is 3 Mbps, Queue 3 can only transmit 3 Mbps and Queue 0 receives the remaining excess bandwidth and can transmit at 6 Mbps.

Note that if there were equal transmit rates explicitly configured, such as 2.5 Mpbs for each queue, the excess bandwidth would be split ebased on the transmit rate (equal in this case), as long as the result in below the shaping rate for the queue.

The second example has a shaping rate (PIR) and transmit rate (CIR) configured on the queues, as shown in Table 80.

Table 80: PIR Mode with Transmit Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 81.

Table 81: PIR Mode with Transmit Rate Hardware Behavior

In this second example, because the transmit rates are less than the shaping rates, each queue will receive their transmit rates.

The third example also has a shaping rate (PIR) and transmit rate (CIR) configured on the queues, as shown in Table 82.

Table 82: Second PIR Mode with Transmit Rate Configuration Example

The way that the IQE PIC hardware interprets these parameters is shown in Table 83.

Table 83: Second PIR Mode with Transmit Rate Hardware Behavior Example

In this third example, all four queues will initially be serviced round-robin. However, Queue 2 has no traffic sent to its queue. So Queue 0, Queue 1, and Queue 3 will all get their respective transmit rates, a total of 9.5 Mbps. The remaining 0.5 Mbps is used by Queue 3, because the transmit rate is the same as the shaping rate. Once this traffic is sent, Queue 0 and Queue 1 will share the excess bandwidth in the ratio of their transmit rates, which total 9 Mbps. In this case, Queue 0 = 5 Mbps + (0.5 Mbps * 5/9) = 5.27 Mbps. Queue 1 = 4 Mbps + (0.5 Mbps * 4/9) = 4.23 Mbps.

The fourth example has a shaping rate (PIR), transmit rate (CIR), and excess rate configured on the queues, as shown in Table 84.

Table 84: PIR Mode with Transmit Rate and Excess Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 85.

Table 85: PIR Mode with Transmit Rate and Excess Rate Hardware Behavior

In this fourth example, all four queues will initially be serviced round-robin. Queue 3 gets 0.5 Mbps of guaranteed bandwidth but cannot transmit more because the shaping rate is the same. Queue 2 has no traffic to worry about at all. Queue 0 and Queue 1 get the respective transmit rates of 3.0 Mbps and 2.5 Mbps. The excess bandwidth of 4 Mbps is divided between Queue 0 and Queue 1 in the ratio on their excess rates. So Queue 1 gets 2.5 Mbps (the guaranteed rate) + 4 Mbps (the excess) + (10% / (50% + 10%)) = 3.17 Mbps. Queue 0 gets the rest, for a total of 6.33 Mbps.

You can configure only an excess rate on the queues, as shown in Table 86.

Table 86: Excess Rate Configuration

The way that the IQE PIC hardware interprets these excess rate parameters is shown in Table 87.

Table 87: Excess Rate Hardware Behavior

In this excess rate example, there are no transmit or shaping rates configured on any of the queues, only excess rates, so bandwidth division happens only on the basis of the excess rates. Note that all the transmit (guaranteed) rates are set to 0. Usually, when there are no excess rates configured, the queue transmit rate is calculated by default. But when there is an excess rate configured on any of the queues, the transmit rate is set to 0. The excess bandwidth (all bandwidths in this case) is shared in the ratio of the excess weights. So Queue 0 receives 10 Mbps * (50 / (50 + 40+ 30+ 20)) = 3.57 Mbps.

It is possible to configure rate limits that result in error conditions. For example, consider the configuration shown in Table 88.

Table 88: PIR Mode Generating Error Condition

The way that the IQE PIC hardware interprets these parameters is shown in Table 89.

Table 89: PIR Mode Generating Error Condition Behavior

In the error example, note that the shaping rates calculated on Queue 2 and Queue 3 are less than the transmit rates on those queues (2.0 Mbps and 0.5 Mbps are eachless than 2.5 Mbps). This is an error condition and results in a syslog error message.

The following set of five examples involve the IQE PIC operating in CIR mode. In CIR mode, the transmit rate and shaping rate calculations are based on the transmit rate of the logical interface. All calculations assume that the logical interface has a shaping rate (PIR) of 20 Mbps and a transmit rate (CIR) of 10 Mbps. The scheduler map has four queues.

The first example has only a shaping rate (PIR) with no excess rate configured on the queues, as shown in Table 90.

Table 90: CIR Mode with No Excess Rate Configuration

Note: The transmit rate (CIR) of 10 Mbps is configured on the logical interface (unit) not the queues in the scheduler map. This is why the queue transmit rates are labeled NA.

The way that the IQE PIC hardware interprets these parameters is shown in Table 91.

Table 91: CIR Mode with No Excess Rate Hardware Behavior

In this first example, all four queues will split the 10-Mbps transmit rate equally and each get a transmit rate of 2.5 Mbps. However, the shaping rate on the interface is 20 Mbps. The 10-Mbps excess bandwidth is divided among the queues in the ratio of their shaping rates. But Queue 2 and Queue 3 are shaped at 3.0 and 4.0 Mbps, respectively, so they cannot use more bandwidth and will get those rates. This accounts for 2 Mbps (the 7 Mbps shaped bandwidth minus the 5 Mbps guaranteed bandwidth for Queue 2 and Queue 3) of the 10-Mbps excess, leaving 8 Mbps for Queue 0 and Queue 1. So Queue 0 and Queue 1 will share the 8-Mbps excess bandwidth in the ratio of their shaping rates, which total 15 Mbps. In this case, Queue 0 = 8.0 Mbps * 8/15 = 4.26 Mbps, for a total of 2.5 Mbps + 4.26 Mbps = 6.76 Mbps. Queue 1 = 8.0 Mbps * 7/15 = 3.73 Mbps, for a total of 2.5 Mbps + 3.73 Mbps = 6.23 Mbps.

The second example has only a few shaping rates (PIR) with no excess rate configured on the queues, as shown in Table 92.

Table 92: CIR Mode with Some Shaping Rates and No Excess Rate Configuration

Note: If a configuration results in the calculated transmit rate of the queue exceeding the shaping rate of the queue, an error message is generated. For example, setting the shaping rate on Queue 2 and Queue 3 in the above example to 20% and 5%, respectively, generates an error message because the calculated transmit rate for these queues (2.5 Mbps) is more than their calculated shaping rates (2.0 Mbps and 0.5 Mbps).

The way that the IQE PIC hardware interprets these parameters is shown in Table 93.

Table 93: CIR Mode with Some Shaping Rates and No Excess Rate Hardware Behavior

In this second example, all four queues will split the 10-Mbps transmit rate equally and each get a transmit rate of 2.5 Mbps. Because of their configured queue shaping rates, Queue 0 and Queue 1 receive preference over Queue 2 and Queue 3 for the excess bandwidth. Queue 0 (8.0 Mbps) and Queue 1 (5.0 Mbps) account for 13 Mbps of the 20 Mbps shaping rate on the logical interface. The remaining 7 Mbps is divided equally between Queue 2 and Queue 3. However, because Queue 3 only has 1 Mbps to send, Queue 2 uses the remaining 6 Mbps.

The third example has shaping rates (PIR) and transmit rates with no excess rate configured on the queues, as shown in Table 94.

Table 94: CIR Mode with Shaping Rates and Transmit Rates and No Excess Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 95.

Table 95: CIR Mode with Shaping Rates and Transmit Rates and No Excess Rate Hardware Behavior

In this third example, the first three queues will get their configured transmit rates and are serviced in round-robin fashion. This adds up to 10 Mbps, leaving a 10-Mpbs excess from the logical interface shaping rate of 20 Mbps. The excess will be shared in the ratio of the transmit rates, or 5:4:1:0. Therefore, Queue 0 will receive 5 Mbps + (5 * 10/10) = 10 Mbps. This value is greater than the 8 Mbps shaping rate on Queue 0, so Queue 0 is limited to 8 Mbps. Queue 1 will receive 4 Mbps + (4 * 10/10) = 8 Mbps. This value is greater than the 5 Mbps shaping rate on Queue 1, so Queue 1 is limited to 5 Mbps. Queue 2 will receive 1 Mbps + (1 * 10/10) = 2 Mbps. This value is equal to the 2 Mbps shaping rate on Queue 2, so Queue 2 receives 2 Mbps. This still leaves 5 Mbps excess bandwidth, and this can be used by Queue 3. Note that this example will never reach the shaping rate configured on the logical interface (20 Mbps).

The fourth example has shaping rates (PIR) and transmit rates with no excess rate configured on the queues. However, in this case the sum of the shaping rate percentages configured on the queues multiplied by the transmit rate configured on the logical interface is greater than the shaping rate configured on the logical interface. The configuration is shown in Table 96.

Table 96: CIR Mode with Shaping Rates Greater Than Logical Interface Shaping Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 97.

Table 97: CIR Mode with Shaping Rates Greater Than Logical Interface Shaping Rate Hardware Behavior

In this fourth example, the first three queues will get their configured transmit rates and are serviced in round-robin fashion. This adds up to 10 Mbps, leaving a 10-Mpbs excess from the logical interface shaping rate of 20 Mbps. The excess will be shared in the ratio of the transmit rates, or 5:4:1:0. Therefore, Queue 0 will receive 5 Mbps + (5 * 10/10) = 10 Mbps. This value is greater than the 8 Mbps shaping rate on Queue 0, so Queue 0 is limited to 8 Mbps. Queue 1 will receive 4 Mbps + (4 * 10/10) = 8 Mbps. This value is greater than the 7 Mbps shaping rate on Queue 1, so Queue 1 is limited to 7 Mbps. Queue 2 will receive 1 Mbps + (1 * 10/10) = 2 Mbps. This value is less than the 5 Mbps shaping rate on Queue 2, so Queue 2 receives 2 Mbps. This still leaves 3 Mbps excess bandwidth, which can be used by Queue 2 (below its shaping rate) and Queue 3 (also below its shaping rate) in the ratio 1:0 (because of the transmit rate configuration). But 1:0 means Queue 3 cannot use this bandwidth, and Queue 2 utilizes 2 Mbps + ( 3 Mbps * 1/1) = 5 Mbps. This is equal to the shaping rate of 5 Mbps, so Queue 2 receives 5 Mbps.

The fifth example has excess rates and transmit rates, but no shaping rates (PIR) configured on the queues. The configuration is shown in Table 98.

Table 98: CIR Mode with Excess Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 99.

Table 99: CIR Mode with Excess Rate Hardware Behavior

In this fifth example, Queue 2 does not have a transmit rate configured. If there were no excess rates configured, then Queue 2 would get a transmit rate equal to the remainder of the bandwidth (3.5 Mbps in this case). However, because there is an excess rate configured on some of the queues on this logical interface, the transmit rate for Queue 2 is set to 0 Mbps. The others queues get their transmit rates and there leaves 13.5 Mbps of excess bandwidth. This bandwidth is divided among Queue 0, Queue 1, and Queue 3 in the ratio of their excess rates. So Queue 0, for example, gets 3.0 Mbps + 13.5 Mbsp * (50 / (50 + 10 + 30)) = 10.5 Mbps.

Four other examples calculating expected traffic distribution are of interest. The first case has three variations, so there are six more examples in all.

• Oversubscribed PIR mode at the logical interface with transmit rates, shaping rates, and excess rates configured at the queues (this example has three variations).
• CIR mode at the logical interface (a non-intuitive case is used).
• Excess priority configured.
• Default excess priority used.

The first three examples all concern oversubscribed PIR mode at the logical interface with transmit rates, shaping rates, and excess rates configured at the queues. They all use a configuration with a physical interface having a shaping rate of 40 Mbps. The physical interface has two logical units configured, logical unit 1 and logical unit 2, with a shaping rate of 30 Mbps and 20 Mbps, respectively. Because the sum of the logical interface shaping rates is more than the shaping rate on the physical interface, the physical interface is in oversubscribed PIR mode. The CIRs (transmit rates) are set to the scaled values of 24 Mbps and 16 Mbps, respectively.

Assume that logical unit 1 has 40 Mbps of traffic to be sent. The traffic is capped at 30 Mbps because of the shaping rate of 30 Mbps. Because the CIR is scaled down to 24 Mbps, the remaining 6 Mbps (30 Mbps – 24 Mbps) qualifies as excess bandwidth.

The following three examples consider different parameters configured in a scheduler map and the expected traffic distributions that result.

The first example uses oversubscribed PIR mode with only transmit rates configured on the queues. The configuration is shown in Table 100.

Table 100: Oversubscribed PIR Mode with Transmit Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 101.

Table 101: Oversubscribed PIR Mode with Transmit Rate Hardware Behavior

The first example has hardware queue transmit rates based on the parent (logical interface unit 1) transmit rate (CIR) value of 24 Mbps. Because there are no excess rates configured, the excess weights are determined by the transmit rates. Therefore, both the logical interface CIR and excess bandwidth are divided in the ratio of the transmit rates. This is essentially the same as the undersubscribed PIR mode and the traffic distribution should be the same. The only difference is that the result is achieved as a combination of guaranteed rate (CIR) and excess rate sharing.

The second example also uses oversubscribed PIR mode, but this time with only excess rate configured on the queues. In other words, the same ratios are established with excess rate percentages instead of transmit rate percentages. In this case, when excess rates are configured, queues without a specific transmit rate are set to 0 Mbps. So the entire bandwidth qualifies as excess at the queue level and the bandwidth distribution is based on the configured excess rates. The expected output rate results are exactly the same as in the first example, except the calculation is based on different parameters.

The third example also uses oversubscribed PIR mode, but with both transmit rates and excess rates configured on the queues. The configuration is shown in Table 102.

Table 102: Oversubscribed PIR Mode with Transmit Rate and Excess Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 103.

Table 103: Oversubscribed PIR Mode with Transmit Rate and Excess Rate Hardware Behavior

The third example has the configured queue transmit rate (CIR) divided according to the ratio of the transmit rates based on the logical interface unit 1 CIR of 25 Mbps. The rest of the excess bandwidth divided according the ratio of the excess rates. The excess 6-Mbps bandwidth is divided equally between Queue 0 and Queue 1 because the excess rates are both configured at 50%. This type of configuration is not recommended, however, because the CIR on the logical interface is a system-derived value based on the PIRs of the other logical units and the traffic distribution at the queue level is based on this value and, therefore, not under direct user control. We recommend that you either configure excess rates without transmit rates at the queue level when in PIR mode, or also define a CIR at the logical interface if you want to configure a combination of transmit rates and excess rates at the queue level. That is, you should use configurations of the CIR mode with excess rates types.

The fourth example uses CIR mode at the logical interface. For this example, assume that a physical interface is configured with a 40-Mbps shaping rate and logical interfaces unit 1 and unit 2. Logical interface unit 1 has a PIR of 30 Mbps and logical interface unit 2 has a PIR of 20 Mbps and a CIR of 10 Mbps. The configuration at the queue level of logical interface unit 1 is shown in Table 104.

Table 104: CIR Mode with Transmit Rate and Excess Rate Configuration

The way that the IQE PIC hardware interprets these parameters is shown in Table 105.

Table 105: CIR Mode with Transmit Rate and Excess Rate Hardware Behavior

The fourth example might be expected to divide the 40 Mbps of traffic between the two logical units in the ratio of the configured transmit rates. But note that because the logical interfaces are in CIR mode, and logical interface unit 1 does not have a CIR configured, the hardware CIR is set to 0 Mbps at the queue level. Bandwidth distribution happens based only on the excess weights. So Queue 0 and Queue 1 get to transmit up to 15 Mbps and 12 Mbps, respectively, while the remaining 3 Mbps is divided equally by Queue 2 and Queue 3.

Note: We recommend configuring a CIR value explicitly for the logical interface if you are configuring transmit rates and excess rates for the queues.

The fifth example associates an excess priority with the queues. Priorities are associated with every queue and propagated to the parent node (logical or physical interface). That is, when the scheduler picks a logical interface, the scheduler considers the logical interface priority as the priority of the highest priority queue under that logical interface. On the IQE PIC, you can configure an excess priority for every queue. The excess priority can differ from the priority used for guaranteed traffic and applies only to traffic in the excess region. The IQE PIC has three “regular” priorities and two excess priorities (high and low, which is the default). The excess priorities are lower than the regular priorities. For more information about configuring excess bandwidth sharing and priorities, see IQE PIC Excess Bandwidth Sharing Configuration.

Consider a logical interface configured with a shaping rate of 10 Mbps and a guaranteed rate of 10 Mbps. At the queue level, parameters are configured as shown in Table 106.

Table 106: Excess Priority Configuration

In this fifth example, Queue 0 is configured with an excess priority of high and all other queues have the default excess priority (low). Bec ause there is no traffic on Queue 2, there is an excess bandwidth of 2.5 Mbps. Because Queue 0 has a higher excess priority, Queue 0 gets the entire excess bandwidth. So the expected output rates on the queues are 4 Mbps + 2.5 Mbps = 6.5 Mbps for Queue 0, 3 Mbps for Queue 1, 0 Mbps for Queue 2, and 0.5 Mbps for Queue 3. Note that this behavior is different than regular priorities. With regular priorities, the transmission is still governed by transmit rates and the priority controls only the order in which the packets are picked up by the scheduler. So without excess configuration, if Queue 0 had a regular priority of high and there was 10 Mbps of traffic on all four queues, the traffic distribution would be 4 Mbps for Queue 0, 3 Mbps for Queue 1, 2.5 Mbps for Queue 2, and 0.5 Mbps for Queue 3 instead of giving all 10 Mbps to Queue 0. Excess priority traffic distributions are governed first by the excess priority and then by the excess rates. Also note that in this example, although the queues are in the excess region because they are transmitting above their configured transmit rates, the logical interface is still within its guaranteed rate. So at the logical interface level, the priority of the queues get promoted to a regular priority and this priority is used by the scheduler at the logical interface level.

The sixth and final example considers the effects of the default excess priority. When the excess priority for a queue is not configured explicitly, the excess priority is based on the regular priority. A regular priority of high maps to an excess priority of high. All other regular priorities map to an excess priority of low. When there is no regular priority configured, the regular and excess priorities are both set to low.

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