The second key topic discussed in this chapter is Multiprotocol Label Switching (MPLS). MPLS is an improved and efficient method for forwarding packets through a network. One of the most important applications of MPLS is in IP+ATM networks. IP+ATM is Cisco's trade name for equipment that simultaneously supports traditional ATM services (PVCs, SVCs, SPVCs, PVPs, and so on), and optimized IP transport using MPLS.
These networks offer traditional ATM and Frame Relay services while providing optimized IP support using ATM MPLS. MPLS also brings important new services, such as IP Virtual Private Networks (VPNs), to both IP+ATM networks and router networks.
IP-based services, such as IP VPNs and VoIP, are increasingly carried on ATM or Frame Relay networks to meet greater demands. Internet service providers (ISPs) are adding traditional Layer 2 capabilities and services, such as traffic engineering and VPNs, to their IP networks. Cisco addresses this problem with MPLS.
Integrating ATM infrastructures into the Internet model is as simple as providing IP continuity between the ATM network and the rest of the IP world. IP is integrated over ATM inside the autonomous system (AS) using MPLS, and the AS is connected to the rest of the Internet via BGP. This is done using IP+ATM, in which the ATM switches can continue to operate according to the ATM Forum and ITU-T standards while running MPLS in parallel. This means that other network applications such as PNNI, SVC, and AutoRoute can still operate independently of the MPLS application offering routed services.
MPLS is increasingly important as the building of internets on ATM expands and develops across the globe. The Internet is a collection of service providers offering IP services to their customers, all interconnected either directly or via high-speed network access points (NAPs). The NAPs are usually managed by a dedicated provider acting as a point of contact for coordination and connectivity purposes.
Each ISP maintains multiple Points of Presence (PoPs) that serve as concentration points for customer connectivity in multiple regions. PoPs can be interconnected via an ATM infrastructure or via direct high-speed leased-line connections. Currently, ISPs use Border Gateway Protocol version 4 (BGP4) for the purposes of interdomain connectivity. BGP4 offers a wide range of capabilities in segmenting providers' networks and offering routing policies that define the providers' administrative and political boundaries.