VPLS is a class of VPN that supports the connection of multiple sites in a single bridged domain over a managed IP/Multi-protocol Label Switching (MPLS) network.
VPLS removes the limitations of ATM and frame relay for providing a protocol-transparent, any-to-any, full-mesh service across a WAN. All services regardless of location in a VPLS appear to be on the same LAN
A VPLS presents an Ethernet interface to enterprises, simplifying the LAN/WAN and enabling rapid and flexible service provisioning. A VPLS uses edge routers that can learn bridge and replicate on a per-VPLS basis. The routers are connected by a full mesh of MPLS Label Switched Path tunnels, enabling any-to-any connectivity. Multiple services can be carried within each LSP tunnel. All services in a VPLS are identified by a unique virtual channel label (pseudo wires), which is exchanged between each pair of edge routers.
Edge routers use these pseudo wires to de-multiplex traffic arriving from different VPLS nodes over the same LSP tunnel. Label switch routers in the path switch traffic based on the outer (transport) label, so the virtual channel label is only visible to the final edge router, where the service terminates.
As traffic arrives on access ports, edge routers collect customers' media access control (MAC) addresses. Each router populates the addresses in a forwarding information base, or table of MAC addresses, it maintains for each VPLS node. All customer traffic is switched according to MAC addresses, and forwarded across the service provider network using the appropriate LSP tunnels.
Because most companies use routers for their WAN connections, the edge routers in a VPLS are exposed only to a single MAC address at each customer location, thus each edge router can scale to support thousands of VPLS services.
Virtual Private LAN Service Benefits
Transparent, protocol-independent, multipoint solution for remote locations – target 3 to 30 locations.
Customers maintain complete control over their own switching and routing between branches, offering easier configuration and debugging in case of problems.
Eliminates L2 protocol conversion between LAN and WAN
Ethernet LAN/WAN interfaces that offer reduction in total cost of ownership
No training required on WAN technologies as in the case with frame relay and ATM
Removes the IP “issues,” namely, trust, security and outsourcing
Bandwidth provisioning on-the-fly for each subscriber independently per VPLS service offered to enterprise
VPLS auto-discovery and service provisioning simplifies addition of new sites without requiring reconfiguration at existing sites.
The Layer 2 Communications Advantage
Modern enterprises to compete effectively need to support key applications that enable collaboration and convergence. Virtual Private LAN Service (VPLS) is generating considerable interest with enterprises as it offers multipoint Ethernet LAN services over MPLS networks. Physicians, attorneys, engineers, architects, and investment managers alike must be able to collaborate and securely access shared information in real-time. In operation a VPLS offers the same connectivity experienced if a device were attached to an Ethernet switch. Increasingly, this information (images, case work, software builds, designs, financial trading data, and more) is stored in the form of large files that do not easily move across a traditional wide area network (WAN). At the same time, real-time converged applications such as voice over IP (VoIP) and videoconferencing that require direct connectivity between offices are running over the WAN.
These services are becoming attractive as many Enterprise applications, utilize peer-to-peer operation that benefit from any-to-any connectivity, which is one of the key attributes of VPLS in that each customer edge device or node communicates directly with all other customer edge nodes associated with the VPLS. VPLS services are "plug and play" in nature meaning that the inherent broadcast nature of Ethernet is used to discover other members connected to the VPLS. As VPLS is based upon Ethernet bridging techniques and is not IP based, the service is often referred to as Transparent LAN service. As many new applications use Layer 2 "heartbeat" mechanisms that cannot be routed, a VPLS allows these applications to be deployed in geographically dispersed locations, which provides enhanced business continuance and availability.
VPLS addresses an important emerging market opportunity to offer Layer 2 multipoint VPN that connects multiple sites. The following types of services may be supported using a VPLS:
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Enterprise customers have requirements to connect multiple corporate sites within a specific metro region at Layer 2. VPLS services enable applications that require Layer 2 connectivity between sites, such as server cluster heartbeats, to be geographically distributed, enhancing business continuity.
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Many small and midsize businesses have non-IP applications such as Microsoft Windows for Workgroups that do use NetBEUI for communications. A VPLS enables these customers to interconnect multiple sites without changing their operating system configurations.
Enterprises that desire any-to-any connectivity between sites, but do not wish to use Layer 3 VPN, may use VPLS delivered via a Layer 2 Communications VPLS solution.
Auto-Discovery and Signaling
An important aspect of VPLS is the ability of network devices to automatically discover and signal to other devices an association with a particular VPN, often referred to as discovery and signaling mechanisms. Within the context of VPLS this includes discovery of other peers associated with a particular VPLS.Older WAN technologies, such as frame relay, were rolled out in the 1990s to support point-to-point and hub-and-spoke networks that interconnect local area networks (LANs) running less-demanding applications. Frame relay was initially designed to handle LAN traffic that was bursty in nature and is suited to processing frames of different length. Enterprises could effectively support these older applications over T1 (1Mbps) and sub-T1 hub-and-spoke frame relay networks.
Frame relay is not efficient, however, at carrying voice and video traffic. That is, any office must be able to connect directly to any office to minimize network delay, rather than 'hub' through another site. Today's LANs are migrating to 100 Mbps and 1 Gbps to support these more demanding applications. Emerging applications such as VoIP are optimized to run in a multipoint-to-multipoint topology.
As a result of these developments, carriers are limiting their investment in their own frame relay networks, and new technologies have emerged to replace frame relay services. Frame-relay end users are faced with the challenge of migrating away from frame relay to newer network technologies that meet their application needs.
Link to VPLS Network Presentation.
WAN Alternatives to Frame Relay
To address the need for more bandwidth, many enterprises use ATM services to interconnect their larger sites (45 Mbps). ATM was developed to provide higher reliability and more scalability than frame relay networks. Hybrid frame relay/ATM networks are commonplace in medium to large enterprises that need to support five to hundreds of sites. Frame relay is used for smaller sites; ATM is used to interconnect larger sites.
Since frame relay and ATM are connection-oriented technologies, they are configured in a way that mimics private lines. A virtual circuit must be manually configured between each site. A three-site network is relatively easy to configure. But to support any-to-any connectivity for a ten-site network, you would need to configure 45 virtual circuits. To support a 100-site network, you would need to configure 4,950 virtual circuits.
Clearly, neither frame relay nor ATM is suited for today's needs. Enterprises have three WAN alternatives to consider in migrating away from frame relay.
IP VPNs
Configure IPSec tunnels over a public or private Internet connection to build a secure and encrypted network. Pros: low cost, ubiquitous coverage. Cons: IP best effort, only support point-to-point and hub-and-spoke topologies.
IP MPLS Services
Use multiprotocol label switching (MPLS) over a public or private Internet connection. Pros: support any-to-any connectivity with CoS/QoS. Cons: costly transition, complex operation.
Ethernet VPLS Services
Use virtual private LAN service (VPLS) over a carrier Ethernet network to provide a WAN that is configured like a LAN or campus area network (CAN). Pros: simple, support any-to-any connectivity with CoS/QoS; high performance; lowest total cost of ownership.
Summary
VPLS provides an architecture that provides Virtual Private LAN Service across geographically dispersed locations using MPLS as a transport. VPLS’s are attractive—they offer a solution to problems that many enterprise customers are seeking to address (high-speed, secure, any-to-any forwarding at Layer 2). The requirement to forward frames at Layer 2 is important, as many new applications and services dictate that the service be transparent to upper-layer protocols (IP) or may lack network layer addressing altogether (NetBEUI).
Link to VPLS Network Presentation.
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