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Understanding Hierarchical Scheduling for MIC and MPC Interfaces


This topic covers the following information:

Scheduler Node Scaling for MIC and MPC Interfaces

In per-unit scheduling, the logical interfaces share a common level 2 node (one per port). In hierarchical-scheduling, each logical interface has its own level 2 node. Thus, scaling is limited by the number of level 2 nodes.

To better control system resources in hierarchical-scheduling mode, you can limit the number of scheduler node levels to two. In this case, all logical interfaces and interface sets with CoS scheduling policy share a single level 2 node. Consequently, the maximum number of logical interfaces with CoS scheduling policies is increased (the interface sets must be at level 3).

To configure scheduler node scaling, include the hierarchical-scheduler statement and set the maximum-hierarchy-levels option to 2 at the [edit interfaces xe-fpc/pic/port] hierarchy level.


The maximum-hierarchy-levels option supports level 3 interface sets but not level 2 interface sets. If you configure level 2 interface sets with the maximum-hierarchy-levels option, you generate Packet Forwarding Engine errors.

Hierarchical Scheduling Priority Levels for MIC and MPC Interfaces

The queuing model used by MIC and MPC interfaces supports three priority levels for guaranteed scheduling priority and two lower priority levels for excess scheduling priority. You can configure a queue with one guaranteed priority and one excess priority. For example, you can configure a queue for guaranteed low (GL) as the guaranteed priority and configure excess high (EH) as the excess priority.

You can associate a guaranteed level with only one excess level. You can associate an excess level with any number of guaranteed priority levels, including none.

Interface nodes maintain their guaranteed priority level (for example, guaranteed high, GH) as long as they do not exceed their guaranteed bandwidth. If the queue bandwidth exceeds the guaranteed rate, then the priority drops to the excess priority (for example, excess high, EH). Because excess level priorities are lower than their guaranteed counterparts, the bandwidth guarantees for each of the other levels can be maintained.

Guaranteed Bandwidth and Weight of an Interface Node on MIC and MPC Interfaces

The queuing model used by MIC and MPC interfaces separates the concepts of guaranteed bandwidth and weight of an interface node, although the two terms are often used interchangeably. The guaranteed bandwidth for an interface node is the bandwidth the node can use, independent of what is happening at the other nodes of the scheduling hierarchy. The weight of an interface node, on the other hand, is a value that determines how excess bandwidth is used. The weight of a node comes into play when other nodes at the same hierarchical scheduling level use less than the sum of their guaranteed bandwidths

For some application traffic types (such as constant bit rate voice, where there is little concern about excess bandwidth), the guaranteed bandwidth dominates the node. For other types of application traffic (such as bursty data, where a well-defined bandwidth is not always possible), the concept of weight dominates the node.

Hierarchical Scheduling for MIC and MPC Interfaces in Oversubscribed PIR Mode

In contrast to the Intelligent Queuing Enhanced (IQE) and Intelligent Queuing 2 Enhanced (IQ2E) PICs, the interfaces on MICs and MPCs set the guaranteed rate to zero in oversubscribed peak information rate (PIR) mode for the per-unit scheduler. Also, the configured rate is scaled down to fit the oversubscribed value. For example, if there are two logical interface units with a shaping rate of 1 Gbps each on a 1-Gbps port (which is, therefore, oversubscribed 2 to 1), then the guaranteed rate on each unit is scaled down to 500 Mbps (scaled down by 2).

With hierarchical schedulers in oversubscribed PIR mode, the guaranteed rate for every logical interface unit is set to zero. This means that the queue transmit rates are always oversubscribed.

Because in oversubscribed PIR mode the queue transmit rates are always oversubscribed, the following are true:

  • If the queue transmit rate is set as a percentage, then the guaranteed rate of the queue is set to zero; but the excess rate (weight) of the queue is set correctly.

  • If the queue transmit rate is set as an absolute value and if the queue has guaranteed high or medium priority, then traffic up to the queue’s transmit rate is sent at that priority level. However, for guaranteed low traffic, that traffic is demoted to the excess low region. This means that best-effort traffic well within the queue’s transmit rate gets a lower priority than out-of-profile excess high traffic. This differs from the IQE and IQ2E PICs.