User Manual

79-6
However, combining the powerful L3 switching function of IP networks and efficient forwarding mechanism of
traditional L2 networks, MPLS uses connection-oriented method at the forwarding plane, similar to the current
L2 network. As a result, it can easily achieve seamless convergence of IP and L2 networks like ATM and
Frame Relay, and provide better solutions for applications like QoS, TE and VPN.
MPLS-based VPN
Traditional VPN transmits private data in the public network via tunnel protocols like GRE, L2TP, and PPTP.
Since LSP is a public network tunnel itself, MPLS is innately advantageous in implementing VPN.
MPLS-based VPN will connect different branches of a private network via LSP to form an integrated one. It
also supports the intercommunication control between different VPN.
Figure 79-4 MPLS-based VPN
The above figure demonstrates the basic structure of MPLS-based VPN: CE (Customer Edge), a router, a
switch or a host; PE (Provider Edge), in the backbone network.
PE manages VPN customers, establishes LSP connections between different PE and distributes routes to
different branches of a VPN customer. The route distribution between PE is usually achieved via LDP or
extended BGP.
MPLS-based VPN supports IP address multiplexing of different branches, and the intercommunication
between different VPN. Different with traditional routes, VPN route contains extra identification of branches
and VPN, making BGP extension a necessity, in order to carry VPN route information.
MPLS-based TE
MPLS-based TE and the Diff-serv feature can provide data flow at different precedent level with different
service while ensuring a high network utility efficiency, and hence, be able to provide low-delay, low packet
loss rate services with a guaranteed bandwidth to various data flows like voice and video.
Considering the difficulty of deploying TE over the whole network, the Diff-serv model is usually the method of
implementing QoS in real networking resolutions.
The basic mechanism of Diff-Serv is mapping a service to a certain service class at the network edge,
according to the required service quality. The service is uniquely identified via the DS segment (originated
from ToS field) of IP packet. According to the segment, the routers in the backbone network will apply
VPN
Branch 2
PE1
PE3
PE2
CE2
Backbone Network
VPN
Branch 3
CE3
VPN
Branch 1
CE1