PLANET XGS3-24042 - Manual (Page 651)

79-5
Figure 79-3 The MPLS Network Structure
The basic working process of MPLS based on the above figure :
First, LDP, together with traditional routing protocols (like OSPF, ISIS, etc) create route tables and LIB (Label
Information Base) for FEC demanding services;
The ingress LER receives packets, completes L3 function, determines the FEC of the packets, labels them,
and thus generates MPLS label packets.
Then, LSR in the network will forward packets according to their labels and LFIB (Label Forwarding
Information Base) without implementing any L3 processing.
Finally, the egress LER of the MPLS will remove the label from the packet before the following IP forwarding.
To sum up, MPLS is neither a service or an application, but a tunnel technology, and a routing and switching
technology platform integrated with label switching forwarding and network layer routing technology. This
platform can support various high-level protocols and services with a certain guarantee of information security
in the transmission.
79.1.3 Introduction to MPLS and Routing Protocols
When LDP creates LSP in hop-by-hop mode, it determines the next-hop based on the information from the
forwarding table of each LSR along it. Since the information from forwarding tables are collected by routing
protocols like IGP and BGP, LDP indirectly relates with them.
Besides, existing protocols like BGP and RSVP, can also distribute MPLS labels after extension.
Sometimes, it is necessary to extend some routing protocols in MPLS applications. For example,
MPLS-based VPN requires extension to BGP, so that, BGP can distribute the VPN (Virtual Private Network)
route information; MPLS-based TE (Traffic Engineering) requires extension to OSPF or IS-IS protocol, to
carry link status information.
79.1.4 MPLS Application Introduction
MPLS technology originally combines L2 switching and L3 routing technology to enhance the route lookup
speed. As ASIC (Application-Specific Integrated Circuit) develops, route lookup speed has no longer been the
bottleneck of network development. As a result, MPLS’s advantage in accelerating forwarding disappears.
LER
MPLS Edge LSR
（ ）
Label Switched Path
( LSP )
Ingress
Egress
MPLS Core LSR