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How CSPF Selects a Path
To select a path, CSPF follows certain rules. The rules are as follows:
- 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 services the LSPs in alphabetical order of the LSP names.
- Prunes the traffic engineering database of all the links that are not full duplex and do not have sufficient reservable bandwidth.
- If the LSP configuration includes the include statement, prunes all links that do not share any included colors.
- If the LSP configuration includes the exclude statement, prunes all links that contain excluded colors. If the link does not have a color, it is accepted.
- If several paths have equal cost, chooses the one whose last-hop address is the same as the LSP’s destination.
- If several equal cost paths remain, selects the one with the fewest number of hops.
- If several equal-cost paths remain, applies the CSPF load-balancing rule configured on the LSP (least fill, most fill, or random).
CSPF finds the shortest path toward 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. All CSPF rules are applied to both computations.
