Virtual Private LAN Service
Virtual Private LAN Service (VPLS) is a way to provide Ethernet based multipoint to multipoint communication over IP/MPLS networks. It allows geographically dispersed sites to share an Ethernet broadcast domain by connecting sites through pseudo-wires. The technologies that can be used as pseudo-wire can be Ethernet over MPLS, L2TPv3 or even Generic Routing Encapsulation (GRE). There are two IETF standards track RFCs (RFC 4761 and RFC 4762) describing VPLS establishment.
VPLS is a virtual private network (VPN) technology. In contrast to L2TPv3, which allows only point-to-point layer 2 tunnels, VPLS allows any-to-any (multipoint) connectivity.
In a VPLS, the local area network (LAN) at each site is extended to the edge of the provider network. The provider network then emulates a switch or bridge to connect all of the customer LANs to create a single bridged LAN.
1. 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.
2. Label stack
VPLS MPLS packets have a two-label stack. The outer label is used for normal MPLS forwarding in the service provider’s network. If BGP is used to establish the VPLS, the inner label is allocated by a PE as part of a label block.