VPLS – Virtual Private LAN Services

Virtual Private LAN Services (VPLS) allows service providers to deliver VPN services based on Ethernet with the same level of support and reliability as existing services such as Frame Relay and ATM. VPLS combines the cost-effectiveness of Ethernet with the service delivery, traffic engineering, scalability and reliability of Multi Protocol Label Switching (MPLS) to provide a multipoint Layer 2 Ethernet Virtual Private Network (VPN) service.

VPNs are a value-added premium service that a service provider can offer to its business customers. VPNs allow service providers to connect multiple customer sites together securely over a common shared network. The service keeps individual customer data private from other customers' data using virtual connections across the shared network. Traditionally, VPNs are based on leased lines, Frame Relay, or ATM services, but more recently on Ethernet, IP and Multi Protocol Label Switching (MPLS).

VPNs are a major source of revenue for service providers as they provide flexibility, scalability and integrate well with data services. However, VPNs use traditional bandwidth delivery services to connect to the customer, and are costly to provision and complex to manage. The continued growth of the Internet and IP-based services combined with a move by customers to outsource more IT functions, such as storage and backup, is placing a strain on these fixed-bandwidth VPN delivery models.

Ethernet provides simple, flexible, more scalable bandwidth than traditional fixed bandwidth solutions and has revolutionised the Metro environment. For the end-user, Ethernet is less expensive, easier to connect and simpler to manage. For the service provider, it is cheaper, more flexible, and allows new services to be provisioned more simply and quickly than traditional solutions.

VPLS - The Best of Both Worlds

MPLS was originally developed as a technology that offers the Layer 2 circuit-switched (conection oriented) characteristics of ATM over a Layer 3 packet switched (connectionless) IP backbone. It allows virtual circuits or tunnels to be created over an IP backbone so that, once set up, data can be switched simply and quickly across the tunnel without the need for further analysis of individual packets or the need to make complicated routing decisions at each network node.

MPLS also provides the quality of service, traffic engineering and availability characteristics of ATM with the scalability, efficiency and multi-point nature of IP.

VPLS was co-developed by Riverstone and others and uses MPLS to allow configuration of multi-point Layer 2 Ethernet VPNs. It uses the Martini encapsulation standard that specifies how technologies like Ethernet, ATM and Frame Relay can be transported over an MPLS network. VPLS empowers service providers Ethernet networks with two key attributes - scalability and availability.

Without VPLS, the scalability of Ethernet networks is limited to the number of unique identifiers or VLAN IDs used to provide services, and the availability of Ethernet networks is limited by the poor resiliency characteristics of mechanisms such as Spanning Tree Protocol. While a number of schemes exist to solve these limitations, such as Q-in-Q stacking and Rapid Spanning Tree, many are proprietary and do not interoperate between vendors.

VPLS solves both of these issues in addition to providing a number of other advantages. A VPLS network can support over a million unique identifiers (MPLS labels). In addition, VPLS specifies dynamic signaling of new paths, allowing traffic from a broken path to be quickly switched to a predetermined backup path, or to a backup path calculated on the fly. For a service provider, VPLS combines the best of both worlds - the cost effectiveness of Ethernet with the scalability and availability of MPLS.

VPLS and BGP MPLS VPNs - A Synergistic Coexistence

Many service providers have implemented Layer 3 BGP MPLS VPNs, also known as IP VPNs, particularly in their backbone IP networks. Service providers contemplating offering VPN services do not have to choose between Layer 2 or Layer 3 MPLS VPN technology. The attributes of both allow them to coexist as each excels in its own realm and provides a different but complementary service.

VPLS is ideally suited to the Metro environment, linking multiple customer sites together in a single Ethernet VPN. VPLS is seen as an ideal replacement for traditional LAN-to-LAN services and a suitable migration for Transparent LAN Services. BGP MPLS VPNs are ideally suited to the backbone environment, linking multiple regional sites together across the WAN.

Traffic from a customer site in one Metro to a customer site in another Metro can be carried over VPLS to the BGP MPLS VPN backbone, which can be configured to carry the traffic between multiple Metros. This type of architecture benefits the service provider as VPLS places less strain on the Metro routers than IP VPN and is simpler and less costly to provision and maintain. BGP MPLS VPNs are best deployed on existing IP backbone routers, improving the utilization of expensive assets and improving return on investment.