LabelSwitching

Label switching is a method in packet-forwarding networks that uses short, fixed-length labels to determine the next hop rather than performing destination-based lookups in the network layer. The most common realization is Multiprotocol Label Switching (MPLS), which allows routers to forward labeled packets through pre-established Label Switched Paths (LSPs).

In operation, an ingress router attaches a stack of labels to a packet. Each label directs the packet to the next hop's forwarder, which may push or pop labels. Routers that perform this forwarding are called Label Switching Routers (LSRs); edge routers capable of creating or removing labels are Label Edge Routers (LERs). The data plane forwards based on an LFIB (or label forwarding table).

Label mapping is distributed by control-plane protocols such as LDP (Label Distribution Protocol) or RSVP-TE (Resource Reservation Protocol with Traffic Engineering). BGP may carry VPN-related label mappings in MPLS VPNs (L3VPN). LSPs may be explicitly configured (TE) or automatically created via signaling.

Benefits include faster forwarding, reduced CPU load on core routers, and enabling traffic engineering and scalable VPNs. It also enables fast re-routing in case of link failure (FRR).

Limitations include increased control-plane complexity, need for compatibility across devices, and potential issues with label imposition and stacking, requiring careful management and security measures.

History: MPLS was developed in the 1990s by IETF as a unifying framework for label switching across IP and other networks; MPLS LDP standardized; it is widely deployed in service provider cores and large enterprise networks.