IS-IS Link State Protocol Data Units with TE Extensions for MPLS Interfaces

An MPLS-enabled device running IS-IS can be configured to send out Link State Protocol (LSP) data units that contain special extensions to support Traffic Engineering (TE). (In this section -- and nowhere else in this chapter -- LSP is the acronym for Link State Protocol. In other sections, LSP means Label Switched Path.) These LSPs are composed of a fixed header and a number of tuples known as Type/Length/Value triplets (TLVs). LSPs that are used for traffic engineering contain a new object called a sub-TLV. Sub-TLVs are similar to regular TLVs except that, where regular TLVs exist inside IS-IS packets, sub-TLVs reside within regular TLVs. Each sub-TLV consists of three fields: a one-octet Type field, a one-octet Length field, and zero or more octets of Value.

These LSPs are flooded throughout the IS-IS domain. LSRs that receive the IS-IS LSPs with TE extensions place the traffic engineering information into a Traffic Engineering Database (TED), which maintains topology data about the nodes and links in the MPLS domain.

IS-IS LSPs have special extensions that contain information related to traffic engineering and are described in RFC 3784. The extensions consist of Type/Length/Value triplets (sub-TLVs) containing the following information:

• IP address of the local interface for the link
• IP address of the remote interface for the link (for point-to-point adjacencies)
• Traffic engineering metric for the link (by default, this is equal to the IS-IS link cost)
• Maximum bandwidth on the interface
• Maximum reservable bandwidth on the interface
• Unreserved bandwidth on the interface
• Administrative groups to which the interface belongs

When configured to do so, the device sends out IS-IS LSPs with TE extensions for each of its MPLS-enabled interfaces. The user can optionally specify the maximum amount of bandwidth that can be reserved on an interface, as well as assign interfaces to administrative groups. Refer to Set Traffic Engineering Parameters for MPLS Interfaces for more information.

Any of the following events trigger the device to send out IS-IS LSPs with a TE extension:

• Change in the interface‘s administrative group membership.
• Change in the interface‘s maximum available bandwidth or maximum reservable bandwidth.
• Significant change in unreserved bandwidth per priority level, which can be either of the following:
  • For any priority level, the difference between the previously advertised, unreserved bandwidth and the current, unreserved bandwidth exceeds five percent of the maximum reservable bandwidth.
  • Any change when the total reserved bandwidth exceeds 95 percent of the maximum reservable bandwidth.

In addition, IS-IS LSPs with TE extensions can be triggered by IS-IS (for example, when an interface‘s link state changes). Furthermore, when an interface is no longer enabled for MPLS, the device stops sending out IS-IS LSPs with TE extensions for that interface.