The Virtual Private LAN Service uses the MPLS network as a transport network and connects local networks via the MPLS WAN. From the point of view of the networked Layer 2 devices, the Virtual Private LAN Service and the underlying MPLS network work like a LAN switch on Layer 2 of the ISO/OSI reference model. Protocols other than IP can also be transmitted via the Virtual Private LAN Service. The Ethernet multipoint connectivity is implemented in the MPLS network. LDP (Label Distribution Protocol) or BGP (Border Gateway Protocol) signal the connections.

The basic operation of MPLS The Virtual Private LAN Service is based on Multiprotocol Label Switching (MPLS). MPLS is a network technology that routes (switches) data traffic on a connection-oriented basis based on predefined paths and labels. The routing decisions are no longer made individually in each IP node than in a connectionless IP network. Routing calculations are only performed once. The labelled data packets then move along the established paths. The labels are part of the four-byte MPLS header. In the OSI reference model, multiprotocol label switching moves between layer three and Layer 2.

MPLS works connection-oriented and allows the transport of IP data traffic with high performance and guaranteed quality parameters. MPLS is not limited to IP and can transmit other protocols. The basic functioning of the Virtual Private LAN Service To explain the basic functionality of the Virtual Private LAN Service, first, an explanation of the two terms PE and CE. CE stands for Customer Edge and designates the system component connected to the MPLS network on the user side (customer side). For example, in the case of VPLS, a CE device is a Layer 2 switch.

The MPLS network and the Virtual Private LAN Service behave transparently for the CE device. The CE device does not have to meet any VPLS-specific requirements and, from its point of view, connects via a switched LAN. The CE devices are connected to provider edge nodes (PE nodes). PE nodes are label edge routers (LER) or ingress and egress label switch routers (LSR). The PE nodes are located at the edge of an MPLS network. VPLS offers Ethernet multipoint connectivity in the direction of CE and maps the full mesh within MPLS using virtual connections and switched label paths. The virtual relationships can be signalled via protocols such as BGP or LDP.

When a PE node receives a LAN frame from a CE device, it checks the MAC destination address of the frame and, with an appropriate label, transmits it via a defined path to the desired destination. Frames with unknown destination addresses or broadcast frames are flooded to all PE nodes of a VPLS mesh. To avoid loops, a split-horizon PE never forwards a frame received from a PE node to a PE node. The label switch routers (EP nodes) are responsible for forwarding the labelled data packets along the established paths within the MPLS network.

Advantages of using the Virtual Private LAN Services The use of the Virtual Private LAN Services offers advantages such as:

• Establishing Layer 2 LAN multipoint connectivity over an MPLS WAN.
• Fully transparent LAN service from the point of view of the CE devices.
• Automatic rerouting within the MPLS in the event of an error – faster failover mechanisms than, for example, the spanning tree protocol.
• Different Layer 2 VPNs and other services can be implemented and operated simultaneously on an MPLS network.
• Flexible allocation of bandwidth and prioritization of traffic.

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