Editing Multiprotocol Label Switching (section)

====Label-switched path====
A label-switched path (LSP) is a path through an MPLS network set up by the [[Network Management System|NMS]] or by a signaling protocol such as [[Label Distribution Protocol|LDP]], [[RSVP-TE]], [[BGP]] (or the now deprecated [[CR-LDP]]). The path is set up based on criteria in the FEC.

The path begins at an LER, which makes a decision on which label to prefix to a packet based on the appropriate FEC. It then forwards the packet along to the next router in the path, which swaps the packet's outer label for another label, and forwards it to the next router. The last router in the path removes 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.

The router which first prefixes the MPLS header to a packet is an [[ingress router]]. The last router in an LSP, which pops the label from the packet, is called an [[egress router]]. Routers in between, which need only swap labels, are called transit routers or label switch routers (LSRs).

Note that LSPs are unidirectional; they enable a packet to be label switched through the MPLS network from one endpoint to another. Since bidirectional communication is typically desired, the aforementioned dynamic signaling protocols can automatically set up a separate LSP in the opposite direction.

When [[link protection]] is considered, LSPs can be categorized as primary (working), secondary (backup) and tertiary (LSP of last resort).