MPLS is not a routing protocol; it works with layer 3 routing protocols (BGP, IS-IS, OSPF) to integrate network layer routing with label switching. An MPLS FEC consists of a set of packets that are all forwarded in the same manner by a given label-switching router (LSR). For example, all packets received on a particular interface might be assigned to a FEC. MPLS assigns each packet to a FEC only at the LSR that serves as the ingress node to the MPLS domain. A label distribution protocol binds a label to the FEC. Each LSR uses the label distribution protocol to signal its forwarding peers and distribute its labels to establish an LSP. The label distribution protocol enables negotiation with the downstream LSRs to determine what labels are used on the LSP and how they are employed.
Labels represent the FEC along the LSP from the ingress node to the egress node. The label is prepended to the packet when the packet is forwarded to the next hop. Each label is valid only between a pair of LSRs. A downstream LSR reached by a packet uses the label as an index into a table that contains both the next hop and a different label to prepend to the packet before forwarding. This table is usually referred to as a label information base (LIB).
The LSR that serves as the egress MPLS node uses the label as an index into a table that has the information necessary to forward the packet from the MPLS domain. The forwarding actions at the egress LSR can be any of the following:
Figure 47 shows a simple MPLS domain, consisting of multiple LSRs. The LSRs serving as ingress and egress nodes are also referred to as label edge routers (LERs). The ingress router is sometimes referred to as the tunnel head end, or the head-end router. The egress router is sometimes referred to as the tunnel tail end, or the tail-end router. LSPs are unidirectional, carrying traffic only in the downstream direction from the ingress node to the egress node.
Figure 47: Simple MPLS Domain
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