MPLS Fundamentals

PCALC

PCALC is the special SPF algorithm that MPLS TE uses. SPF is an algorithm that OSPF and IS-IS 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 ...