MPLS VPNs
One of the most popular of the MPLS applications is called MPLS virtual private networks
(VPNs). MPLS VPNs allow a service provider, or even a large enterprise, to offer Layer 3 VPN
services. In particular, SPs oftentimes replace older Layer 2 WAN services such as Frame Relay
and ATM with an MPLS VPN service. MPLS VPN services enable the possibility for the SP to
provide a wide variety of additional services to its customers because MPLS VPNs are aware of
the Layer 3 addresses at the customer locations. Additionally, MPLS VPNS can still provide the
privacy inherent in Layer 2 WAN services.
MPLS VPNs use MPLS unicast IP forwarding inside the SP’s network, with additional MPLSaware
features at the edge between the provider and the customer. Additionally, MPLS VPNs use
MP-BGP to overcome some of the challenges when connecting an IP network to a large number
of customer IP internetworks—problems that include the issue of dealing with duplicate IP
address spaces with many customers.
This section begins by examining some of the problems with providing Layer 3 services and then
shows the core features of MPLS that solve those problems.