A VPLS is a provider-provisioned service that emulates the full functionality of a traditional LAN. Using VPLS to connect customer sites over an MPLS network identifies the basic components of a VPLS.

Figure 1. Using VPLS to connect customer sites over an MPLS network

A VPLS makes the remote LAN segments behave as one single LAN. CE devices that belong to a specific VPLS appear to be on a single bridged Ethernet.

Two separate VPLS standards, RFC 4761 and RFC 4762, define almost identical approaches to implementing the VPLS data plane, but define significantly different approaches to implementing the VPLS control plane. RFC 4761 recommends BGP signaling, and RFC 4762 recommends LDP signaling.

Both standards extend the Martini VPN capabilities by defining a new circuit type for Ethernet VLAN. The new circuit type enables VPLS to support 802.1Q VLANs, traditional Ethernet encapsulation, and broadcast and multicast functions.

Although the hub-and-spoke topology type is considered, both standards recommend the use of a full mesh topology for all members of a VPLS. For the full mesh topology, each PE must run a Forwarder instance for each individual VPLS that the PE supports.

In a VPLS, a Forwarder makes forwarding decisions by examining the destination MAC addresses, or destination MAC addresses and VLAN tags, in a data packet’s Layer 2 header. In essence, a Forwarder operates as a virtual Layer 2 bridge or switch. As such, each Forwarder must maintain a routing table so that it can forward an incoming user data packet to the correct pseudowire.