How CSPF calculates a traffic-engineered path
Using information in the TED in addition to the attributes and requirements of the LSP, CSPF calculates a traffic-engineered path for the LSP by performing the tasks listed below.
- When more than one LSP needs to be enabled, CSPF selects the LSP for path calculation based on the LSPs setup priority and bandwidth requirement.
- Eliminate unsuitable links from consideration.
The device examines the topology information in its TED and uses this information to eliminate links from consideration for the traffic-engineered path. A link is eliminated when any of the following are true:
- The link does not have enough reservable bandwidth to fulfill the LSPs configured requirements.
- The LSP has an include statement, and the link does not belong to an administrative group in the statement.
The LSP has an exclude statement, and the link belongs to an administrative group specified in the exclude statement.
- Using the remaining links, calculate the shortest path through the MPLS domain.
Using the links that were not eliminated in the previous step, the device calculates the shortest path between the ingress and egress LERs. When the LSP is configured to use an explicit path, the device individually calculates the shortest path between each node in the path.
By default, the path calculated by CSPF can consist of no more than 255 hops, including the ingress and egress LERs. You can optionally change this maximum to a lower number.
- When multiple paths have the same cost, select one of them.
The output of the CSPF process is a traffic-engineered path, a sequential list of the physical interfaces that packets assigned to this LSP pass through to reach the egress LER. Once the traffic-engineered path has been determined, RSVP signaling attempts to establish the LSP on each LSR in the path.