3Com S7902E, Configuration Manual

Page: 1439 / 2621

1-5
Figure 1-4
Structure of the MPLS network
Ingress
LSP
Egress
Transit
IP network
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 Information Base (LFIB)
for the next hop according to the label of the packet, swaps the label, and then 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 of the packet and IP forwards the
packet.
Obviously, MPLS is not a service or application, but actually a tunneling technology and a routing and
switching technology platform that combines label switching with Layer 3 routing. This platform not only
supports multiple upper layer protocols and services, but also secures transmission of information to a
certain degree.