There is often a great deal of confusion in understanding the different “flavours” of MPLS networks. I recently read a post online by a gentleman named Mbong Ekwoge who wrote a rather clear online posting:
MPLS is the enabler of all these fancy services and applications we hear about today, such as MPLS VPNs, AToM (Any Transport over MPLS), MPLS TE (Traffic Engineering), etc.
In order to clearly understand what VPLS is, you need to understand what led to the “birth” of VPLS (Virtual Private LAN Service). It all began with MPLS VPNs. The client had to form a peer-to-peer relationship with the Provider’s PE routers. What this means is that the provider is intricately involved with routing and forwarding the customer’s traffic and some customers did not like this idea. Also, providers had invested heavily into Layer 2 VPN techniques such as ATM, Frame Relay, etc and completely eliminating these overlay VPN techniques didn’t feel right with their financial people. Some engineers did not like the idea of having to let go of their beloved ATMs, Frame Relay PVCs for some new chap coming in.
This led Cisco and IETF to develop a solution which would let you run MPLS in the core but users will still maintain their private Layer-2 VPN service across the MPLS core of the service provider. What this means is, the provider will provide a VPN service, across MPLS, but it will be kind of a pseudowire experience. The customer still retains their highly valued privacy, the Service Provider maintains her MPLS core and should the customer be convinced, transitioning to MPLS VPNs will be like “bread and butter”.
Now this led to the introduction of AToM. AToM is the Cisco name for the Layer 2 transport service over an MPLS backbone. The customer routers interconnect with the service provider routers at Layer 2 (Ethernet, High-Level Data Link Control [HDLC], PPP, ATM, or Frame Relay). This eliminates the need for the legacy network from the service provider carrying these kinds of traffic and integrates this service into the MPLS network that already transports the MPLS VPN traffic.
AToM is an open standards-based architecture that uses the label switching architecture of MPLS and can be integrated into any network that is running MPLS. The advantage to the customer is that they do not need to change anything. Their routers that are connecting to the service provider routers can still use the same Layer 2 encapsulation type as before and do not need to run an IP routing protocol to the provider edge routers as in the MPLS VPN solution. As such, the move from the legacy network that is running ATM or Frame Relay to the network that is running AToM is completely transparent to the customer. The service provider does not need to change anything on the provider (P) routers in the core of the MPLS network. The intelligence to support AToM sits entirely on the PE routers. As such, the core and edge technologies (MPLS and AToM, respectively) are decoupled. The core label switching routers (LSRs) only switch labeled packets, whereas the edge LSRs impose and dispose of labels on the Layer 2 frames. This is similar to the MPLS VPN solution, in which the P routers switch only labeled packets and the PE routers need the intelligence to impose and dispose of labels on the IP VPN traffic from the customers.
Now how does VPLS come into the equation????
AToM is a point-to-point service and hence cannot broadcast frames.
Now some technologies such as Ethernet are broadcast in nature and take for example, the Spanning Tree Protocol (STP). These protocols operate in a broadcast nature. VPLS is the point-to-multipoint cousin of AToM.
