Label-switched paths (LSPs) are unidirectional routes through a network or autonomous system (AS). In normal IP routing, the packet has no predetermined path. Instead, each router forwards a packet to the next-hop address stored in its forwarding table, based only on the packet's destination address. Each subsequent router then forwards the packet using its own forwarding table.
In contrast, MPLS routers within an AS determine paths through a network through the exchange of MPLS traffic engineering information. Using these paths, the routers direct traffic through the network along an established route. Rather than selecting the next hop along the path as in IP routing, each router is responsible for forwarding the packet to a predetermined next-hop address.
Figure 1 shows a typical LSP topology.
Figure 1: Typical LSP Topology
In the topology shown in Figure 1, traffic is forwarded from Host C1 to the transit network with standard IP forwarding. When the traffic enters the transit network, it is switched across a preestablished LSP through the network. In this example, an LSP might switch the traffic from Router R4 to Router R2 to Router R1. When the traffic exits the network, it is forwarded to its destination by IP routing protocols.