The MPLS VPN Control Plane
The MPLS VPN control plane defines protocols and mechanisms to overcome the problems
created by overlapping customer IP address spaces, while adding mechanisms to add more functionality
to an MPLS VPN, particularly as compared to traditional Layer 2 WAN services. To understand
the mechanics, you need a good understanding of BGP, IGPs, and several new concepts
created by both MP-BGP RFCs and MPLS RFCs. In particular, this section introduces and
explains the concepts behind three new concepts created for MPLS VPNs:
■ VRFs
■ Route Distinguishers (RDs)
■ Route Targets (RTs)
The next several pages of text examine these topics in order. While reading the rest of the MPLS
VPN coverage in this chapter, note that the text will keep expanding a single example. The
example focuses on how the control plane learns about routes to the duplicate customer subnets
10.3.3.0/24 on the right side of Figure 19-12, puts the routes into the VRFs on PE2, and advertises
the routes with RDs over to PE1 and then how RTs then dictate how PE1 adds the routes to its
VRFs.