Hierarchical CoS for Metro Ethernet Environments

In metro Ethernet environments, a virtual LAN (VLAN) typically corresponds to a customer premises equipment (CPE) device and the VLANs are identified by an inner VLAN tag on Ethernet frames (called the customer VLAN, or C-VLAN, tag). A set of VLANs can be grouped at the DSL access multiplexer (DSLAM) and identified by using the same outer VLAN tag (called the service VLAN, or S-VLAN, tag). The service VLANs are typically gathered at the Broadband Remote Access Server (B-RAS) level. Hierarchical schedulers let you provide shaping and scheduling at the service VLAN level as well as other levels, such as the physical interface. In other words, you can group a set of logical interfaces and then apply scheduling and shaping parameters to the logical interface set as well as to other levels.

You can apply CoS shaping and scheduling at one of four different levels, including the VLAN set level. (Some devices support up to five levels of scheduler hierarchies.)

The supported scheduler hierarchy is as follows:

• The physical interface (level 1)

• The service VLAN (level 2)

• The logical interface or customer VLAN (level 3)

• The queue (level 4)

Users can specify a traffic control profile (output-traffic-control-profile) that can specify a shaping rate, a guaranteed rate, and a scheduler map with transmit rate and buffer delay. The scheduler map contains the mapping of queues (forwarding classes) to their respective schedulers (schedulers define the properties for the queue). Queue properties can specify a transmit rate and buffer management parameters such as buffer size and drop profile.

To configure CoS hierarchical scheduling, you must enable hierarchical scheduling by including the hierarchical-scheduler statement at the physical interface.