Hierarchical Class of Service Overview

HCoS is implemented primarily using traffic classifiers at the ingress and hierarchical schedulers and shapers at the egress.

A classifier is a filter that labels traffic at the device ingress based on configurable parameters such as application or destination. Traffic is classified into what is called a forwarding equivalence class (FEC). The FEC defines a class of traffic that receives common treatment.

Schedulers, and their associated shapers, are the functions that control the traffic bandwidth, jitter (delay variation), and packet loss priority at the egress of the device.

Hierarchical schedulers are used to apply multiple levels of scheduling and shaping with each level applied to different classifications such as forwarding equivalence class, VLAN, and physical interface (port) as shown in Figure 1.

Figure 1: Hierarchical Scheduling Architecture

A typical application of HCoS is to configure multiple levels of egress schedulers and shapers, at the subscriber edge, using dynamic profiles to provide traffic shaping and prioritization at the subscriber VLAN level and for multiple classes of traffic.

Dynamic profiles are a mechanism that allows you to dynamically apply schedulers and shapers to individual subscribers or groups of subscribers.

To learn more about HCoS, the following topics are very helpful:

• Junos CoS on MX Series 5G Universal Routing Platforms Overview

• CoS Features and Limitations on MX Series Routers

• CoS Features of the Router Hardware, PIC, MIC, and MPC Interface Families

• How Schedulers Define Output Queue Properties

• Subscriber Access Network Overview

• CoS for Subscriber Access Overview

• Hierarchical Class of Service for Subscriber Management Overview

The Junos OS hierarchical schedulers support up to five levels of scheduler hierarchies on MX Series devices when using enhanced queuing Dense Port Concentrators (DPCs) or fine-grained queuing Modular Port Concentrators (MPCs), and Modular Interface Cards (MICs). It is important to know the capabilities of your hardware with respect to HCoS. The following are a few tips to help you:

• Only certain hardware supports the five-level scheduler hierarchy of HCoS.

• The number of queues and logical interfaces supported is dependent upon exactly what hardware you are using.

• The MX Series Packet Forwarding Engine handles guaranteed bandwidth and scheduler node weight differently than other Packet Forwarding Engines.

• The fine-grained queuing MPCs and MICs have a certain granularity with respect to the shaping and delay buffer values. The values used are not necessarily exactly the values configured.

To learn more about platform support for HCoS, use the Juniper Networks Feature Explorer (https://pathfinder.juniper.net/feature-explorer/). In the Feature Explorer, search on hierarchical schedulers.

In addition, it is important to note the following:

• HCoS is most frequently used to enforce service level agreements at the subscriber edged using dynamic traffic control profiles.

• Hierarchical schedulers can also be applied to Ethernet pseudowire interfaces, aggregated Ethernet interfaces, Layer 2 Tunnel Protocol (L2TP) network server (LNS) inline services, and GRE tunnels.

• Hierarchical ingress policing is a feature that is complimentary to and often used in conjunction with HCoS.

• There are other features in Junos OS that have similar sounding names.

Before planning HCoS for you network, you should learn about HCoS, define you needs, plan how you want to implement HCoS, and test the operation in a simulated environment.

Documentation related to HCoS is consolidated in the Hierarchical Class of Service User Guide.