MPLS Terminology

Table 21-1 defines the terminology used with MPLS implementations.

Table 21-1. MPLS Terminology

Term	Definition
Label Switch Router (LSR)	The core device that switches labeled packets according to precomputed switching tables. This device also can be a switch or a router.
Label	The header used by an LSR to forward packets. The header format depends on network characteristics. In router networks, the label is a separate, 32-bit header. In ATM networks, the label is placed into the Virtual Path Identifier /Virtual Channel Identifier (VPI/VCI) cell header. In the core, LSRs read only the label, not the network layer packet header. One key to the scalability of MPLS is that labels have only local significance between two devices that are communicating. Local significance means that only the sender of the traffic cares what the labels are, not the receiver. For example, if a letter is received in your mailbox #17, you don't care that the postmaster put the same letter in the post office's outgoing mailbox #42.
Edge Label Switch Router (Edge LSR) or Label Edge Router (LER)	The edge device that performs initial packet processing and classification and applies the first label. This device can be either a router such as the Cisco 7500 or a switch with built-in routing, such as the Cisco MGX 8800.
Label Switched Path (LSP)	The path defined by all labels assigned between end points. An LSP can be dynamic or static. Dynamic LSPs are provisioned automatically using routing information. Static LSPs are explicitly provisioned.
Label Distribution Protocol (LDP)	Communicates labels and their meaning among LSRs. It assigns labels in edge and core devices to establish LSPs in conjunction with routing protocols such as Open Shortest Path First (OSPF), Intermediate System to Intermediate System (IS-IS), Routing Information Protocol (RIP), Enhanced Interior Gateway Routing Protocol (EIGRP), or Border Gateway Protocol (BGP).

Figure 21-2 illustrates the relationship between LSRs, LSPs, and Edge LSRs/LERs.

Figure 21-2. MPLS Components