On IQE PICs, scheduling occurs level-by-level. That is, based on the parameters configured on the logical interface, the scheduler first picks a logical interface to transmit from. Then, based on the configuration of the underlying queues, the IQE PIC selects one of the queues to transmit from. Therefore, it is important to understand how different logical interface parameters are configured or derived (not explicitly configured), and also how the same values are established at the queue level.
In the following examples, assume that the bandwidth available at the physical interface level is 400 Mbps and there are four logical interfaces (units) configured. A per-unit scheduler is configured, so the logical interfaces operate in different modes depending on the parameters configured.
If no class-of-service parameters are configured on any of the logical interfaces, the interface is in default mode. In default mode, the guaranteed rate (CIR) available at the physical interface (400 Mbps) is divided equally among the four logical interfaces. Each of the four will get a guaranteed rate (CIR) of 100 Mbps. Because none of the four logical interfaces have a shaping rate (PIR) configured, each logical interface can transmit up to the maximum of the entire 400 Mbps. Because there is no excess rate configured on any of the logical interfaces, each of the four gets an equal, minimum excess weight of 1. The configured and hardware-derived bandwidths for this default mode example are shown in Table 72.
Table 72: Default Mode Example for IQE PICs
If a subset of the logical interfaces (units) have a shaping rate (PIR) configured, but none of them have a guaranteed rate (CIR) or excess rate, then the physical interface is in PIR mode. Furthermore, if the sum of the shaping rates on the logical interfaces is less than or equal to the physical interface bandwidth, the physical interface is in undersubscribed PIR mode. If the sum of the shaping rates on the logical interfaces is more than the physical interface bandwidth, the physical interface is in oversubscribed PIR mode. These modes are the same as on other PICs, where only a shaping rate and guaranteed rate can be configured.
In undersubscribed PIR mode, the logical interfaces with a configured shaping rate receive preferential treatment over those without a configured shaping rate. For logical interfaces with a shaping rate configured, the guaranteed rate is set to the shaping rate. For the logical interfaces without a shaping rate, the remaining logical interface bandwidth is distributed equally among them. Excess weights for the logical interfaces with a shaping rate are set to an implementation-dependent value proportional to the shaping rate. Excess weights for the logical interfaces without a shaping rate are set to the minimum weight (1). However, although the excess weights for the configured logical interfaces are never used because the logical interfaces cannot transmit above their guaranteed rates, the excess weights are still determined for consistency with oversubscribed mode. Also, logical interfaces without a configured shaping rate can transmit up to a maximum of the physical bandwidth of the other queues that are not transmitting. Therefore, the shaping rate (PIR) is set to the physical interface bandwidth on these interfaces.
The configured and hardware-derived bandwidths for the undersubscribed PIR mode example are shown in Table 73. Note that the sum of the shaping rates configured on the logical interfaces (500 Mbps) is more than the physical interface bandwidth (400 Mbps).
Table 73: Undersubscribed PIR Mode Example for IQE PICs
In the oversubscribed PIR mode, where the sum of the configured shaping rates on the logical interfaces exceeds the physical interface bandwidth, we cannot set the guaranteed rate to the shaping rate because this might result in the sum of the guaranteed rates exceeding the physical interface bandwidth, which is not possible. In this mode, we want the logical interfaces with shaping rates configured to share the traffic proportionally when these logical interfaces are transmitting at full capacity. This could not happen if the guaranteed rate was set to the shaping rate. Instead, in hardware, we set the guaranteed rates to a “scaled down” shaping rate, so that the sum of the guaranteed rates of the logical interfaces do not exceed the physical interface bandwidth. Because there is no remaining bandwidth once this is done, the other logical interfaces receive a guaranteed rate of 0. Excess weights are set proportionally to the shaping rates and for logical interfaces without a shaping rate, the excess weight is set to a minimum value (1). Finally, the shaping rate is set to the shaping rate configured on the logical interface or to the physical interface bandwidth otherwise.
The configured and hardware-derived bandwidths for the oversubscribed PIR mode example are shown in Table 74. Note that the sum of the shaping rates configured on the logical interfaces (300 Mbps) is less than the physical interface bandwidth (400 Mbps).
Table 74: Oversubscribed PIR Mode Example for IQE PICs
If none of the logical interfaces have an excess rate configured, but at least one of the logical interfaces has a guaranteed rate (CIR) configured, then the physical interface is in CIR mode. In this case, the guaranteed rates are set in hardware to the configured guaranteed rate on the logical interface. For logical interfaces that do not have a guaranteed rate configured, the guaranteed rate is set to 0. The hardware shaping rate is set to the value configured on the logical interface or to the full physical interface bandwidth otherwise. The excess weight is calculated proportional to the configured guaranteed rates. Logical interfaces without a configured guaranteed rate receive a minimum excess weight of 1.
The configured and hardware-derived bandwidths for the CIR mode example are shown in Table 75. In CIR mode, the shaping rates are ignored in the excess weight calculations. So although logical unit 1 has an explicitly configured PIR and logical unit 3 does not, they both receive the minimum excess weight of 1.
Table 75: CIR Mode Example for IQE PICs
If one of the logical interfaces has an excess rate configured, then the physical interface is in excess rate mode. Strictly speaking, this mode only matters for the calculation of excess weights on the logical interface. The hardware guaranteed and shaping rates are determined as described previously. In excess rate mode, the excess weights are set to a value based on the configured excess rate. Logical interfaces which do not have excess rates configured receive a minimum excess weight of 1.
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Note: Because the excess rate only makes sense above the guaranteed rate, you cannot configure an excess rate in PIR mode (PIR mode has only shaping rates configured). You must configure at least one guaranteed rate (CIR) on a logical interface to configure an excess rate. |
The excess rate is configured as a percentage in the range from 1 through 100. The configured value is used to determine the excess weight in the range from 1 through 127.
The configured and hardware-derived bandwidths for the excess rate mode example are shown in Table 76. When an excess rate is configured on one or more logical interfaces, the shaping rate and the guaranteed rate are both ignored in the excess weight calculations. So logical unit 2 gets a minimum excess weight of 1, even though it has a guaranteed rate configured.
Table 76: Excess Rate Mode Example for IQE PICs
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