3Com MSR 50 Series Configuration Manual page 1315

Figure 380 Structure of the MPLS network
Ingress
IP network
The following describes how MPLS operates:
1 First, the LDP protocol and the traditional routing protocol (such as OSPF and ISIS)
work together on each LSR to establish the routing table and the label information
base (LIB) for intended FECs.
2 Upon receiving a packet, the ingress LER completes the Layer 3 functions,
determines the FEC to which the packet belongs, labels the packet, and forwards
the labeled packet to the next hop along the LSP.
3 After receiving a packet, each transit LSR looks up its label forwarding table for the
next hop according to the label of the packet and forwards the packet to the next
hop. None of the transit LSRs performs Layer 3 processing.
4 When the egress LER receives the packet, it removes the label from the packet and
performs IP forwarding.
Obviously, MPLS is not a service or application, but actually a tunneling technology
and a routing and switching technology platform combining label switching with
Layer 3 routing. This platform supports multiple upper layer protocols and services,
as well as secure transmission of information to a certain degree.
LSP
Transit
MPLS Overview 1315
Egress
IP network