MPLS Routers
MPLS defines two roles for routers. A Label Switch Router (LSR) has all its
interfaces within the MPLS network and does its path selection primarily
based on labels. An Edge LSR has some interfaces in the MPLS network and
some in a normal IP network, and so does some routing and some label
switching. An LSR is sometimes referred to as a provider (P) router, and the
edge LSR as a provider edge (PE) router.
LSRs function at two planes, the control plane and the data plane. The
control plane handles routing protocols and a label-exchange protocol called
Label Distribution Protocol (LDP). It contains the routing table and the
Label Information Base (LIB). The data plane contains the CEF FIB and
adjacency table and the MPLS Label FIB (LFIB); it forwards traffic based
on those. Figure 3-1 shows the functions at each plane.