Constrained Shortest Path First

Whereas IGPs use the Shortest Path First (SPF) algorithm to determine how traffic is routed within a network, RSVP uses the Constrained Shortest Path First (CSPF) algorithm to calculate traffic paths that are subject to the following constraints:

• LSP attributes—Administrative groups such as link coloring, bandwidth requirements, and EROs
• Link attributes—Colors on a particular link and available bandwidth

These constraints are maintained in the traffic engineering database (TED). The database provides CSPF with up-to-date topology information, the current reservable bandwidth of links, and the link colors.

In determining which path to select, CSPF follows these rules:

1. Computes LSPs one at a time, beginning with the highest-priority LSP—the one with the lowest setup priority value. Among LSPs of equal priority, CSPF starts with those that have the highest bandwidth requirement.
2. Prunes the traffic engineering database of links that are not full duplex and do not have sufficient reservable bandwidth.
3. If the LSP configuration includes the include statement, prunes all links that do not share any included colors.
4. If the LSP configuration includes the exclude statement, prunes all links that contain excluded colors. If a link does not have a color, it is accepted.
5. Finds the shortest path toward the LSP's outbound router, taking into account any EROs. For example, if the path must pass through Router A, two separate SPF algorithms are computed: one from the inbound router to Router A and one from Router A to the outbound router.
6. If several paths have equal cost, chooses the one with a last-hop address the same as the LSP's destination.
7. If several equal-cost paths remain, selects the path with the fewest number of hops.
8. If several equal-cost paths remain, applies CSPF load-balancing rules configured on the LSP.