Understanding Hierarchical Scheduling

Hierarchical Scheduling Terminology

Hierarchical scheduling introduces some new CoS terms and also uses some familiar terms in different contexts:

• Customer VLAN (C-VLAN)—A C-VLAN, defined by IEEE 802.1ad. A stacked VLAN contains an outer tag corresponding to the S-VLAN, and an inner tag corresponding to the C-VLAN. A C-VLAN often corresponds to CPE. Scheduling and shaping is often used on a C-VLAN to establish minimum and maximum bandwidth limits for a customer. See also S-VLAN.

• Interface set—A logical group of interfaces that describe the characteristics of set of service VLANs, logical interfaces, customer VLANs, or aggregated Ethernet interfaces. Interface sets establish the set and name the traffic control profiles. See also Service VLAN.

• Scheduler— A scheduler defines the scheduling and queuing characteristics of a queue. Transmit rate, scheduler priority, and buffer size can be specified. In addition, a drop profile may be referenced to describe WRED congestion control aspects of the queue. See also Scheduler map.

• Scheduler map—A scheduler map is referenced by traffic control profiles to define queues. The scheduler map establishes the queues that comprise a scheduler node and associates a forwarding class with a scheduler. See also Scheduler.

• Stacked VLAN—An encapsulation on an S-VLAN with an outer tag corresponding to the S-VLAN, and an inner tag corresponding to the C-VLAN. See also Service VLAN and Customer VLAN.

• Service VLAN (S-VLAN)—An S-VLAN, defined by IEEE 802.1ad, often corresponds to a network aggregation device such as a DSLAM. Scheduling and shaping is often established for an S-VLAN to provide CoS for downstream devices with little buffering and simple schedulers. See also Customer VLAN.

• Traffic control profile—Defines the characteristics of a scheduler node. Traffic control profiles are used at several levels of the CLI, including the physical interface, interface set, and logical interface levels. Scheduling and queuing characteristics can be defined for the scheduler node using the shaping-rate, guaranteed-rate, and delay-buffer-rate statements. Queues over these scheduler nodes are defined by referencing a scheduler map. See also Scheduler and Scheduler map.

• VLAN—Virtual LAN, defined on an Ethernet logical interface.

Scheduler Node-Level Designations in Hierarchical Scheduling

Scheduler hierarchies are composed of nodes and queues. Queues terminate the hierarchy. Nodes can be either root nodes, leaf nodes, or internal (non-leaf) nodes. Internal nodes are nodes that have other nodes as “children” in the hierarchy.

Scheduler hierarchies consist of levels, starting with Level 1 at the physical port. This chapter establishes a four-level scheduler hierarchy which, when fully configured, consists of the physical interface (Level 1), the interface set (Level 2), one or more logical interfaces (Level 3), and one or more queues (Level 4).

Table 1 describes the possible combinations of scheduler nodes and their corresponding node level designations for a hierarchical queuing MIC or MPC.

The table illustrates how the configuration of an interface set or logical interface affects the terminology of hierarchical scheduler nodes. For example, suppose you configure an interface-set statement with logical interfaces (such as unit 0 and unit 2) and a queue. In this case, the interface-set is an internal node at Level 2 of the scheduler node hierarchy. However, if there are no traffic control profiles attached to logical interfaces, then the interface set is at Level 3 of the hierarchy.

Hierarchical Scheduling at Non-Leaf Nodes

Whereas standard CoS scheduling is based on the scheduling and queuing characteristics of a router’s egress ports and their queues, hierarchical CoS scheduling is based on the scheduling and queuing characteristics that span a hierarchy of scheduler nodes over a port. The hierarchy begins at Level 1, a root node at the physical interface (port) level of the CLI hierarchy and terminates at Level 4, a leaf node at the queue level. Between the root and leaf nodes of any scheduler hierarchy are one or more internal nodes, which are non-root nodes that have other nodes as “children” in the hierarchy.

Whereas you configure standard CoS scheduling by applying a scheduler map to each egress port to specify a forwarding class and a queue priority level, you configure hierarchical CoS scheduling with additional parameters. To configure hierarchical CoS scheduling, you apply a scheduler map to the queue level (Level 4) of a scheduler hierarchy, and you can apply a different traffic control profile at each of the other levels. A traffic control profile specifies not only a scheduler map (forwarding class and queue priority level) but also optional shaping rate (PIR), guaranteed transmit rate (CIR), burst rate, delay buffer rate, and drop profile.