Note: Many topics at this site are reduced versions of the text in "The Encyclopedia of Networking and Telecommunications." Search results will not be as extensive as a search of the book's CD-ROM.

Constraint-based routing is a QoS (Quality of Service) routing technique that has become important with the development of MPLS (Multiprotocol Label Switching). However, RFC 2702 (Requirements for Traffic Engineering Over MPLS, September 1999) notes that QoS routing is really a subset of a more broadly defined constraint-based routing approach.

MPLS is a protocol for carrier-based core networks that runs over MPLS-enabled IP routers and ATM switches. Such devices are called MPLS LSRs (label switch routers). An MPLS network permits the definition of explicit paths, which are predefined routes through networks as opposed to routes that are selected at each router on a hop-by-hop basis. Routing protocols such as OSPF and BGP determine these explicit routes in advance, and then build tables in each router that define the routes. Packets carry labels to indicate which explicit route they should be taking. Thus, labeled packets follow LSPs (label switched paths).

The preceding procedure of using standard routing protocols to define explicit paths is really the default procedure, and it can take place without operator intervention. In addition, MPLS is flexible enough to define paths based on various constraints such as available bandwidth, the priority setting of packets, the whims of an operator, or the directives of a policy-based server. Thus, MPLS also supports CBR (constraint-based routing).