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Multiclass LSP Configuration

Multiclass LSP Overview

A multiclass LSP is an LSP that can carry several class types. One multiclass LSP can be used to support up to four class types. On the packets, the class type is specified by the EXP bits (also known as the class-of-service bits) and the per-hop behavior (PHB) associated with the EXP bits. The mapping between the EXP bits and the PHB is static, rather than being signaled in RSVP.

Once a multiclass LSP is configured, traffic from all of the class types can:

  • Follow the same path

  • Be rerouted along the same path

  • Be taken down at the same time

Class types must be configured consistently across the Differentiated Services domain, meaning the class type configuration must be consistent from router to router in the network.

You can unambiguously map a class type to a queue. On each node router, the CoS queue configuration for an interface translates to the available bandwidth for a particular class type on that link.

The combination of a class type and a priority level forms a traffic engineering class. The IGPs can advertise up to eight traffic engineering classes for each link.

For more information about the EXP bits, see MPLS Label Allocation.

For more information about forwarding classes, see the Junos OS Class of Service User Guide for Routing Devices.

Multiclass LSPs

Multiclass LSPs function like standard LSPs, but they also allow you to configure multiple class types with guaranteed bandwidth. The EXP bits of the MPLS header are used to distinguish between class types. Multiclass LSPs can be configured for a variety of purposes. For example, you can configure a multiclass LSP to emulate the behavior of an ATM circuit. An ATM circuit can provide service-level guarantees to a class type. A multiclass LSP can provide a similar guaranteed level of service.

The following sections discuss multiclass LSPs:

Establishing a Multiclass LSP on the Differentiated Services Domain

The following occurs when a multiclass LSP is established on the differentiated services domain:

  1. The IGPs advertise how much unreserved bandwidth is available for the traffic engineering classes.

  2. When calculating the path for a multiclass LSP, CSPF is used to ensure that the constraints are met for all the class types carried by the multiclass LSP (a set of constraints instead of a single constraint).

  3. Once a path is found, RSVP signals the LSP using an RSVP object in the path message. At each node in the path, the available bandwidth for the class types is adjusted as the path is set up. The RSVP object is a hop-by-hop object. Multiclass LSPs cannot be established through routers that do not understand this object. Preventing routers that do not understand the RSVP object from carrying traffic helps to ensure consistency throughout the differentiated services domain by preventing the multiclass LSP from using a router that is incapable of supporting differentiated services.

By default, multiclass LSPs are signaled with setup priority 7 and holding priority 0. A multiclass LSP configured with these values cannot preempt another LSP at setup time and cannot be preempted.

It is possible to have both multiclass LSPs and regular LSPs configured at the same time on the same physical interfaces. For this type of heterogeneous environment, regular LSPs carry best-effort traffic by default. Traffic carried in the regular LSPs must have the correct EXP settings.