When you configure a signaled Label Switched Path, you specify the address of the egress LER, as well as optional attributes, such as the LSPs priority and bandwidth requirements.

When you enable the signaled LSP, the Constrained Shortest Path First (CSPF) process on the ingress LER uses this information to calculate a traffic-engineered path between the ingress and egress LERs. You can optionally specify a path of LSRs that the LSP must pass through on the way to the egress LER.

CSPF is an advanced form of the Shortest Path First (SPF) process used by IGP routing protocols. The CSPF process on the ingress LER uses the configured attributes of the LSP, user-specified path (when there is one), and the information in the Traffic Engineering Database (TED) to calculate the traffic-engineered path. This process consists of a sequential list of the physical interfaces that packets assigned to this LSP pass through to travel from the ingress LER to the egress LER. The traffic-engineered path takes into account the network topology, available resources, and user-specified constraints. The traffic-engineered path calculated by CSPF may or may not be the same as the shortest path that would normally be calculated by standard IGP routing protocols.

CSPF is enabled by default for signaled LSPs.

Once the path for the LSP has been calculated, RSVP signaling then causes resources to be reserved and labels to be allocated on each LSR specified in the path. This may cause already existing, lower priority LSPs to be preempted. Once resources are reserved on all the LSRs in the path, the signaled LSP is considered to be activated; that is, packets can be forwarded over it.

The following sections provide additional information about the individual components of the process for activating traffic-engineered signaled LSPs.