MPLS L3VPN Overview

This section covers these topics:

• Introduction to MPLS L3VPN
• MPLS L3VPN Concept.
• MPLS L3VPN Packet Forwarding
• MPLS L3VPN Networking Schemes
• MPLS L3VPN Routing Information Advertisement
• Multi-AS VPN
• Carrier’s Carrier
• Nested VPN
• Multi-Role HostHoVPN
• OSPF VPN Extension
• BGP AS Number Substitution

Introduction to MPLS L3VPN

MPLS L3VPN is a kind of PE-based L3VPN technology for service provider VPN solutions. It uses BGP to advertise VPN routes and uses MPLS to forward VPN packets on service provider backbones.MPLS L3VPN provides flexible networking modes, excellent scalability, and convenient support for MPLS QoS and MPLS TE. Hence, it is widely used.

The MPLS L3VPN model consists of three kinds of devices:

• Customer edge device (CE): A CE resides on a customer network and has one or more interfaces directly connected with service provider networks. It can be a router, a switch, or a host. It neither can "sense" the existence of any VPN nor needs to support MPLS.
• Provider edge router (PE): A PE resides on a service provider network and connects one or more CEs to the network. On an MPLS network, all VPN processing occurs on the PEs.
• Provider (P) router: A P router is a backbone router on a service provider network. It is not directly connected with any CE. It only needs to be equipped with basic MPLS forwarding capability.

The MPLS L3VPN model.

Network diagram for MPLS L3VPN mode

• CEs and PEs mark the boundary between the service providers and the customers.
• A CE is usually a router. After a CE establishes adjacency with a directly connected PE, it advertises its VPN routes to the PE and learns remote VPN routes from the PE. 
• A CE and a PE use BGP/IGP to exchange routing information. You can also configure static routes between them.
• After a PE learns the VPN routing information of a CE, it uses BGP to exchange VPN routing information with other PEs. 
• A PE maintains routing information about only VPNs that are directly connected, rather than all VPN routing information on the provider network.
• A P router maintains only routes to PEs. It does not need to know anything about VPN routing information.
• When VPN traffic travels over the MPLS backbone, the ingress PE functions as the ingress LSR, the egress PE functions as the egress LSR, while P routers function as the transit LSRs.