Virtual Private LAN Service - Mesh Establishment

Mesh Establishment

Since VPLS emulates a LAN, full mesh connectivity is required. There are two methods for full mesh establishment for VPLS: using BGP and using Label Distribution Protocol (LDP). The "control plane" is the means by which provider edge (PE) routers communicate for auto-discovery and signaling. Auto-discovery refers to the process of finding other PE routers participating in the same VPN or VPLS. Signaling is the process of establishing pseudo-wires (PW). The PWs constitute the "data plane", whereby PEs send customer VPN/VPLS traffic to other PEs.

With BGP, one has auto-discovery as well as signaling. The mechanisms used are very similar to those used in establishing Layer-3 MPLS VPNs. Each PE is configured to participate in a given VPLS. The PE, through the use of BGP, simultaneously discovers all other PEs in the same VPLS, and establishes a full mesh of pseudo-wires to those PEs.

With LDP, each PE router must be configured to participate in a given VPLS, and, in addition, be given the addresses of other PEs participating in the same VPLS. A full mesh of LDP sessions is then established between these PEs. LDP is then used to create an equivalent mesh of PWs between those PEs.

An advantage to using PWs as the underlying technology for the data plane is that in case of failure, traffic will automatically be routed along available backup paths in the service provider's network. Failover will be much faster than could be achieved with e.g. Spanning Tree Protocol (STP). VPLS is thus a more reliable solution for linking together Ethernet networks in different locations than simply connecting a WAN link to Ethernet switches in both locations.

VPLS has significant advantages for both service providers and customers. Service providers benefit because they can generate additional revenues by offering a new Ethernet service with flexible bandwidth and sophisticated service level agreements (SLAs). VPLS is also simpler and more cost effective to operate than a traditional service. Customers benefit because they can connect all of their sites to an Ethernet VPN that provides a secure, high speed and homogenous network. Moreover, VPLS provides a logical next step in the continuing evolution of Ethernet from a 10 Mbit/s shared LAN protocol to a multi-Gbps global service.