PCALC is the special SPF algorithm that MPLS TE uses. SPF is an algorithm that OSPF and ISIS use to calculate the shortest path to a destination. In short, SPF runs on every router and uses the database built by OSPF or IS-IS to distill a routing table. The only criterion that is important to SPF is the lowest cost for each IP prefix. For TE, however, other criteria play a role: the resources or constraints on the links. Because OSPF and IS-IS have been extended to distribute these resources, too, PCALC can calculate a path not only based on shortest path but also based on these resources. The SPF algorithm becomes a CSPF algorithm. Basically, links that do not have sufficient bandwidth or that do not have the right resources are pruned off while the SPF tree is built. Another difference with regular SPF is that PCALC or CSPF can run upon request— namely, when you configure a TE tunnel. The result of a CSPF calculation is not a routing table, but a path. This path is an explicit route, which is nothing other than a sequence of IP addresses, where each IP address represents one interface on a router. This explicit route is then used to set up or signal the TE LSP. PCALC builds exactly one path for the TE tunnel, never two or more. Therefore, it is important to know what the best path is in case multiple possible paths are presented. If multiple paths have the same cost and adhere to all the constraints, the path with the largest minimum bandwidth is chosen. If multiple paths are still possible, the path that has the fewest hops is chosen. If multiple paths still remain, Cisco IOS picks one for you.

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