labelswitchingpath

A label switched path (LSP) is a unidirectional path through a Multiprotocol Label Switching (MPLS) network along which labeled data packets are forwarded from an ingress Label Switching Router (LSR) to an egress LSR. An LSP is defined by a sequence of LSRs and a corresponding stack of labels that determine how packets are switched as they traverse the network.

Operation in brief: At the ingress LSR, packets receive an MPLS label stack (push). Each intermediate LSR consults its label forwarding information base (LFIB) to swap the top label for a new one, continuing the forwarding along the path. The egress LSR pops the label stack (pop) and hands the packet to the final IP destination. This label-based forwarding allows data to bypass traditional IP routing for the core portion of the path, enabling faster switching and easier implementation of services such as traffic engineering and VPNs.

Establishment and management: LSPs are typically established using label distribution protocols. LDP (Label Distribution Protocol) provides basic label distribution for standard MPLS forwarding, while RSVP-TE (Resource Reservation Protocol - Traffic Engineering) supports explicit routing and bandwidth reservation for traffic-engineered LSPs. LSPs can be static (manually configured) or dynamic (signaled by LDP or RSVP-TE). LSPs may be protected or restored using techniques such as fast reroute for fault tolerance.

Usage and benefits: LSPs enable scalable, label-based forwarding that separates data plane decisions from IP routing, facilitating QoS, traffic engineering, VPNs, and efficient multicast. They also support rapid failure recovery and simplified core routing.

See also: MPLS, LSR, LDP, RSVP-TE, FEC (Forwarding Equivalence Class), LFIB, FRR.