ervice) A multipoint virtual private network (VPN) service from carriers that connects any number of Ethernet LANs together over an IP core, typically using MPLS, although other encapsulation protocols can be used. Known as a "transparent LAN service" (TLS), from a customer's viewpoint, all connections appear as one Ethernet network, and all protocols in the customer's network (IP, IPX, etc.) are automatically supported.
Prior to VPLS, attempts at transparent LAN services had been problematic. Ethernet is a broadcasting technology, while carrier networks are point-to-point. VPLS was designed to resolve this dichotomy and provide ease of scalability for the service provider.
The Provider Creates the Mesh
Connecting two LANs together can be easily done using a private line or frame relay, however, three or more LANs requires provisioning private lines or frame relay virtual circuits between each site. For example, in a frame relay VPN, four sites would need 12 unidirectional circuits. With VPLS, the customer leases one circuit for each site, and the service provider's network creates the mesh of connections over IP.
LANs Connect to Edge Routers
Each LAN connects to a VPLS edge router that looks up the destination Ethernet address (MAC address) and adds a virtual channel label to the packet. The edge routers provide a special form of packet replication in order to support Ethernet's broadcasting architecture. Packets are directed through the Internet via MPLS label switch routers (LSRs) to the edge router at the other end. Service providers can serve multiple customers from a single location by multiplexing several streams of data from that location to the same edge router. See virtual private network
Frame Relay VPN Vs. VPLS Services
Frame relay services have to be provisioned with virtual circuits for each site-to-site connection. VPLS customers lease connections from each location to the carrier, and the MPLS fabric handles the routing. (Illustration drawn with assistance courtesy of Alcatel, www.alcatel.com)