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IS-IS Extensions to Support Traffic Engineering

To help provide traffic engineering and MPLS with information about network topology and loading, extensions have been added to the Junos OS implementation of IS-IS. Specifically, IS-IS supports new type, length, and value (TLV) tuples that specify link attributes. These TLVs are included in the IS-IS link-state PDUs. The link-attribute information is used to populate the traffic engineering database, which is used by the Constrained Shortest Path First (CSPF) algorithm to compute the paths that MPLS label-switched paths (LSPs) take. This path information is used by RSVP to set up LSPs and reserve bandwidth for them.


Whenever possible, use IS-IS interior gateway protocol (IGP) shortcuts instead of traffic engineering shortcuts.

The traffic engineering extensions are defined in RFC 5305, IS-IS Extensions for Traffic Engineering.

IS-IS IGP Shortcuts

In IS-IS, you can configure shortcuts, which allow IS-IS to use an LSP as the next hop as if it were a subinterface from the ingress routing device to the egress routing device. The address specified in the to statement at the [edit protocols mpls label-switched-path lsp-path-name] hierarchy level must match the router ID of the egress routing device for the LSP to function as a direct link to the egress routing device and to be used as input to IS-IS SPF calculations. When used in this way, LSPs are no different than Asynchronous Transfer Mode (ATM) and Frame Relay virtual circuits (VCs), except that LSPs carry only IPv4 traffic.