When it comes to MPLS, undoubtedly virtual private network (VPN) is one of its most popular and successful applications. Any Transport over MPLS (AToM) or L2VPN as it’s sometimes referred to allows customers’ sites to be interconnected at a Layer 2 level over the carrier’s MPLS backbone whilst supporting numerous different access technologies such as Frame Relay, ATM, PPP and Ethernet. Although some of these are considered legacy and are rarely encountered in modern network deployments, they still exist in some production environments scattered across the globe.
A large scale Service Providers (SP) has invested a lot of money over years in different network infrastructures and sold thousands of circuits to customers needing connectivity. Their customers in return might have invested a lot of time and money deploying/migrating equipment to support these technologies which at the time from a business perspective was the most feasible solution. With the great success of MPLS VPN (L3VPN), such SPs have migrated their core network or backbone to MPLS, however, some legacy access circuits and infrastructures remained. Numerous customers still rely on these and have very little incentive to change to anything else. AToM allows SPs to take advantage of a unified core infrastructure–as opposed to separate networks for different technologies running alongside each other– whilst at the same time providing support to legacy and current Layer 2 access technologies used by their customers. This process is completely transparent to customers who aren’t required to change anything and still connect to the SP using exactly the same Layer 2 access circuit and encapsulation type as before.
Another Layer 2 transport technology is also available called Layer 2 Tunnelling Protocol version 3 (L2TPv3). In essence it has the same architecture as AToM, however, the underlying network carrying the service is different. Whilst AToM carries traffic over a MPLS backbone, L2TPv3 does so over an IP only backbone. The transported Layer 2 frames are encapsulated into IP packets with an L2TPv3 header as opposed to MPLS labels.
Essentially we have two major models when it comes to L2VPNs, Virtual Private Wire Services (VPWS) for Layer 2 point-to-point circuits and Virtual Private LAN Service (VPLS) for Layer 2 point-to-multipoint services. AToM–and L2TPv3 for that matter– falls under VPWS as they’re limited to creating Layer 2 point-to-point circuits known as pseudowires. It is also considered an edge technology restricted to the Provider Edge (PE). As such, the core and edge technologies are decoupled in a much similar way MPLS VPN decouples the need of P routers knowing individual customers’ routes. The P routers are essentially swapping or popping transport labels whereas the PE routers are the ones imposing or disposing the appropriate VPN label making the entire process transparent to core routers which are unaware VPN traffic is being transported.
Here’s a diagram to illustrate the AToM pseudowire emulation concept.
In the above diagram, a tunnel is created over the IP/MPLS backbone between PEs and inside it one or more pseudowires may exist connecting remote customers Access/Attachment Circuits (AC) to each other. As as example of a traffic flow, CE1 sends a frame over its directly connected AC. PE1 encapsulates the received frame and sends it across the appropriate pseudowire to PE2 which in turn extracts and forwards the frame towards CE2. This emulated tunnel results in the CE devices seeing themselves as directly connected despite the fact a pseudowire exist between them.