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Understanding Two-Level and Three-level Hierarchical CoS for Subscriber Interfaces on MX Series Routers

Hierarchical CoS enables you to apply traffic scheduling and queuing parameters and packet transmission scheduling parameters to an individual subscriber interface rather than to all interfaces configured on the port. Hierarchical CoS is supported on MX Series routers with either EQ DPCs or MPC/MICs installed.

On Juniper Networks MX Series routers, MPC/MIC and EQ DPC interfaces support a four-level CoS scheduling hierarchy that, when fully configured, consists of the physical interface (level 1), an interface set or underlying interface (level 2), one or more logical interfaces (level 3), and one or more queues (level 4). Although all CoS scheduling hierarchies are four-level, level 1 is always the physical interface and level 4 is always the queue. Hierarchical scheduling configurations consist of the type of interfaces you configure; for example, a logical interface or an interface set and where those interfaces reside in the scheduling hierarchy, either level 2 or level 3. Because many hierarchical scheduling configurations are possible, we use the terms two-level hierarchical scheduling and three-level hierarchical scheduling in this discussion.

Two-Level Hierarchical Scheduling

Two-level hierarchical scheduling limits the number of hierarchical levels in the scheduling hierarchy to two (level 2 and level 3) as shown in Figure 1. In this configuration, interface sets are not configured and only the logical interfaces have traffic control profiles. Two-level hierarchical scheduling is supported on MX Series routers running either MPC/MIC or EQ DPC interfaces.

Figure 1: Two-Level Hierarchical Scheduling

In a two-level scheduling hierarchy, all logical interfaces and interface sets share a single level 2 node; no hierarchical relationship is formed.

You control two-level hierarchical scheduling by including the maximum-hierarchy-levels option under the [edit interfaces interface-name hierarchical-scheduler] statement:

• When the maximum-hierarchy-levels option is not set, interface sets can be at either level 2 or level 3, depending on whether the member logical interfaces within the interface set have a traffic control profile.
• If any member logical interface has a traffic control profile, then the interface set is always a level 2 CoS scheduler node.
• If no member logical interface has a traffic control profile, the interface set is always a level 3 CoS scheduler node.
• If the maximum-hierarchy-levels option is set, then the interface set can only be at level 3; it cannot be at level 2. In this case, if you configure a level 2 interface set, you generate Packet Forwarding Engine errors.

Table 1 summarizes the interface hierarchy and the CoS scheduler node levels for two-level hierarchical scheduling.

To configure two-level hierarchical scheduling, include the hierarchical-scheduler statement at the [edit interfaces interface-name] hierarchy level. You can optionally include the maximum-hierarchy-levels option. If you choose to set this option, the only supported value is 2.

Three-Level Hierarchical Scheduling

Three-level hierarchical scheduling is supported only on MX Series routers running MPC/MIC interfaces. Three-level hierarchical scheduling has up to eight class of service queues. You can configure many different three-level scheduling hierarchies, depending on the location of the interface set and the use of underlying interfaces. In all variations, the physical interface is a level 1 CoS scheduler node and the queues reside at level 4.

Note: Three-level hierarchical scheduling is supported only on subscriber interfaces and interface sets running over aggregated Ethernet interfaces on MPC/MIC interfaces in MX Series routers.

When you use three-level hierarchical scheduling, interface sets can reside at either level 2 or level 3. You can also configure an underlying logic interface at level 2 and a logical interface at level 3. Table 2 summarizes the most common cases of the interface hierarchy and the CoS scheduler node levels for three-level hierarchical scheduling.

In three-level hierarchical scheduling, the CoS scheduler nodes at level 1, level 2, and level 3 form a hierarchical relationship; this differs from two-level hierarchical scheduling where no hierarchical relationship is formed.

With a three-level hierarchical scheduling, logical interfaces can reside at level 2, or they can reside at level 3, if the logical interface at level 2 is an underlying logical interface. This is shown in Figure 2.

Figure 2: Three-Level Hierarchical Scheduling: Logical Interfaces at Level 3 with Underlying Logical Interfaces at Level 2

Another possible configuration for three-level hierarchical scheduling is shown in Figure 3. In this configuration, the logical interfaces are located at level 2 and the interface sets are located at level 3.

Figure 3: Three-Level Hierarchical Scheduling: Logical Interfaces at Level 2 with Interface Sets at Level 3

To configure three-level hierarchical scheduling, include the implicit-hierarchy option at the [edit interfaces interface-name hierarchical-scheduler] hierarchy level.

Interface Hierarchy Versus CoS Hierarchy

An interface hierarchy and a CoS scheduling hierarchy are distinctly different. Interface hierarchy refers to the relationship between the various interfaces; for example, the relationship between logical interfaces and an interface set, the relationship between a logical interface and an underlying logical interface, or the relationship between the physical interface and logical interface. CoS scheduling hierarchy refers to the hierarchical relationship between the CoS scheduler nodes. In two-level hierarchical scheduling, no hierarchy is formed between the CoS scheduler nodes; all logical interfaces and interface sets share a single level 2 scheduler node. However, when you use the implicit-hierarchy option for three-level hierarchical scheduling, the CoS scheduler nodes form a scheduling hierarchy.

Figure 4 and Figure 5 provide two scenarios for this discussion. Figure 4 shows an interface hierarchy where a Gigabit Ethernet interface (GE-1/0/0) is the physical interface. Two logical interfaces (GE-1/0/0.100 and GE-1/0/0.101) are configured on the physical interface:

• Logical interface GE-1/0/0.100 is a member of a PPPoE interface set and a Demux interface set.
• Logical interface GE-1/0/0.101 is a member of a demux interface set.

Figure 4: Logical Interfaces at Level 2 and Interface Sets at Level 3

Each interface set has a dedicated queue. The CoS scheduler nodes at level 1 (physical interface), level 2 (underlying logical interfaces), and level 3 (interface sets) form a scheduling hierarchy.

To configure this scenario, you must include the implicit-hierarchy option under the hierarchical-scheduler statement on physical interface GE-1/0/0 and configure and apply traffic control profiles on each interface set and underlying logical interface.

Figure 5 shows an interface hierarchy where Gigabit Ethernet interface GE-1/0/0 is the physical interface. Three logical interfaces are configured:

• Two logical interfaces (Pp0.100 and Demux0.100) reside on the underlying logical interface GE-1/0/0.100.
• A third logical interface (Pp0.101) resides on the underlying logical interface GE-1/0/0.101.

Figure 5: Logical Interfaces at Level 3 and Underlying Logical Interfaces at Level 2

Each logical interface has a dedicated queue. The CoS scheduler nodes at level 1 (physical interface), level 2 (underlying logical interfaces), and level 3 (logical interfaces) form a scheduling hierarchy.

To configure this scenario, you must include the implicit-hierarchy option under the hierarchical-scheduler statement on physical interface GE-1/0/0 and configure and apply traffic control profiles on each logical interface and underlying logical interface.

You can configure many different three-level scheduling hierarchies; Figure 4 and Figure 5 present just two possible scenarios. Table 2 summarizes the possible interface locations and CoS scheduler nodes.