Hierarchical Schedulers Terminology
Hierarchical schedulers introduce some new terms into a discussion of CoS capabilities. They also use some familiar terms in different contexts. This section presents a complete overview of the terms used with hierarchical schedulers.
The following terms are important for hierarchical schedulers:
- 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.
These terms are especially important when applied to a scheduler hierarchy. Scheduler hierarchies are composed of nodes and queues. Queues terminate the CLI 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. For example, if an interface-set statement is configured with a logical interface (such as unit 0) and queue, then the interface-set is an internal node at Level 2 of the hierarchy. However, if there are no traffic control profiles configured on logical interfaces, then the interface set is at Level 3 of the hierarchy.
Table 1 shows how the configuration of an interface set or logical interface affects the terminology of hierarchical scheduler nodes.
Table 1: Hierarchical Scheduler Nodes
Root Node (Level 1) | Level 2 | Level 3 | Queue (Level 4) |
|---|---|---|---|
Physical interface | Interface set | Logical interfaces | One or more queues |
Physical interface | Interface set | One or more queues | |
Physical interface | Logical interfaces | One or more queues |
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).
