As you might have guessed, LSP stands for label-switched path, and TSP stands for tagged-switched path. As shown in the example below, this is the path your label-switched packet will take through your network.
Here are a few facts on LSP/TSPs
- The LSP is unidirectional, meaning it is only in one direction; an LSP must be built for return traffic.
- When the LSP is built, it uses information from the IP routing protocol to determine the best path to take to the destination Forward Equivalency Class (FEC)
- An FEC is how we describe packets forwarded based on a common characteristic; for example, with IP routing, the FEC would be the destination address, or with QoS, the FEC could be a QoS value.
- The LSP can take a different path from the path chosen by the IP routing protocol. This is implemented when you use MPLS Traffic Engineering.
- You do not want to use aggregation if you run MPLS VPNs or MPLS TE. This will break the LSP, and if a labeled packet gets to an aggregation point, the P router, which does not have the customer routing table, will not know how to forward the labeled packet, and by default, the packet will be dropped.