A label-switched path (LSP) is a unidirectional path through the MPLS network. You can set up an LSP using any of the signaling protocols such as LDP, RSVP, or BGP. The path begins at an ingress provider edge (PE) switch, which makes a decision on which label to prefix to a packet based on the appropriate forwarding equivalence class (FEC). FECs are a set of packets that have identical characteristics (they use the same next hop, interface) and are to be forwarded in a similar way.
The PE switch then forwards the packet along to the next switch (a provider switch) in the path, which swaps or pops the packet outer label for another label, and forwards it to the next switch. The action—pop or swap—that a provider switch performs is determined by the position of the switch in the LSP. The penultimate provider switch or the last switch (the egress provider edge switch) in the path removes (pops) the label from the packet and forwards the packet based on the header of its next layer (for example IPv4). Due to the forwarding of packets through an LSP being opaque to higher network layers, an LSP is also sometimes referred to as an MPLS tunnel.
For more information, click Junos OS MPLS for EX Series.