MPLS & VPLS: The Wedding

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). Now, 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 buy this idea and 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 the Chief Accountants and CIOs. 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 L2 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 SP maintains her MPLS core and should the customer be convinced, transitioning to MPLS VPNs will be "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.
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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 Protocoo (STP). These protocols operate in a broadcast nature. VPLS is the point-to-multipoint cousin of AToM.