rcuit. This binding process can be changed at any time SO that a customer port can easily be mapped to one context and then remapped to another with only one line of configuration code. GRE routers can carry multicast traffic in GRE tunnels. Carriers can, therefore, distribute multicast traffic, such as video, in GRE VPNs or use GRE tunnels to carry multicast traffic through a unicast network. The latter application is ideal when a carrier wants to support multicast capability in only certain select routers or segments of the network. A provider can support GRE VPNs as only the first step in its VPN strategy. If the provider wants to eventually offer Border Gateway Protocol (BGP) and MPLS VPNs, it can continue to offer GRE VPNs while the net- work is migrated to MPLS and the BGP mesh between provider edge routers is con- figured. Then the provider can migrate GRE VPN customers to be BGP MPLS VPNs (which are discussed later in this chapter, in the section "RFC 2547 (BGP MPLS) VPNs"). Layer 3 VPNs The objective of Layer 3 VPNs is to virtualize multiple per-VPN forwarding instances within a single physical platform. This offers new customer IP VPN ser- vices, reduces capital and operational expenditures, offers scalability, and provides per-control plane protection, isolation, and security. The provider forwards pack- ets based on customer Layer 3 addresses, and the customer does less routing because the provider assists in distributing customer routes to VPN sites. The basic components of a Layer 3 VPN are the provider, provider edge, and cus- tomer edge routers. The provider routers are IP or MPLS routers found in the core, and they interconnect the provider edge routers at the edge. A provider edge router sits at the edge of the provider's network and provides the interface between the cus- tomer edge router and the IP or MPLS backbone. The customer edge routers exchange their routing tables with the provider edge routers via standard routing protocols, such as RIP, OSPF, Enhanced Interior Gateway Routing Protoco