The devices that participate in the MPLS Protocol mechanisms can be classified into label-edge routers (LERs) and label-switching routers (LSRs). An LSR is a high-speed router device in the core of an MPLS network that participates in the establishment of LSPs using the appropriate label signaling protocol and high-speed switching of the data traffic based on the established paths.
An LER is a device that operates at the edge of the access network and MPLS network. LERs support multiple ports connected to dissimilar networks (such as frame relay, ATM, and Ethernet) and forward this traffic on to the MPLS network after establishing LSPs. LERs use the label-signaling protocol at the ingress and distribute the traffic back to the access networks at the egress. LERs play an important role in the assignment and removal of labels, as traffic enters or exits an MPLS network.
the Internet Engineering Task Force (IETF) established the Multi-protocol Label Switching (MPLS) working group to produce a unified and interoperable multilayer switching standard as each vendor (Cisco Systems, Lucent, and so on) developed a proprietary multilayer switching solution, maintaining the IP control component and label-swapping components in different ways. The majority of these multilayer switching solutions required an ATM transport because they could not operate over mixed media infrastructures, such as Frame Relay, PPP, SONET, and LANs.