LSRs

LSRs is an acronym that can refer to several concepts depending on the field. The term most commonly appears in computer networking in connection with MPLS, where it denotes a Label Switched Router.

In MPLS networks, an LSR is a router that forwards packets based on short path labels rather than IP addresses. It maintains a forwarding table, the LFIB, which maps inbound labels to actions such as swap, push, or pop of labels and the next hop. An LSR receives labeled packets from neighboring LSRs or from Label Edge Routers (LERs), consults its LFIB, and forwards the packet toward its destination along a label-switched path (LSP). LSRs rely on a label distribution protocol (LDP) or RSVP-TE to establish and maintain the set of labels used across the network.

In the MPLS architecture, LERs sit at the edge of the network, pushing labels for forward traffic into the core, while core routers perform transit switching. LSRs can be integrated with traditional IP routing and switch traffic at high speed, enabling traffic engineering and improved performance. Features such as penultimate hop popping (PHP) optimize label popping by removing the last label at the penultimate hop.

Beyond networking, LSR can denote other terms in different domains, but these uses are context-dependent and far less standardized. In networking, LSR is the most established meaning. See also Label Edge Router, Label Switched Path, and Label Distribution Protocol.