How CSPF Selects a Path

To select a path, CSPF follows these steps:

1. Compute 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. Prune the topology database (TED) of all the links that are not full duplex and do not have sufficient reservable bandwidth.
3. If the LSP configuration includes the include statement, prune all links that do not share any included colors.
4. If the LSP configuration includes the exclude statement, prune all links that contain excluded colors and do not contain a color.
5. Find the shortest path towards the LSP's egress router, taking into account explicit-path constraints. For example, if the path must pass through Router A, two separate SPFs are computed, one from the ingress router to Router A, the other from Router A to the egress router.
6. If several paths have equal cost, choose the one whose last hop address is the same as the LSP's destination.
7. If several equal-cost paths remain, select the one with the fewest number of hops.
8. If several equal-cost paths remain, apply the CSPF load-balancing rule configured on the LSP (least-fill, most-fill, or random).