ITPRC News - April, 2002
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ITPRC NEWS - April, 2002 -

What is a "Layer 2 VPN?"
By Irwin Lazar

Day three of last week's MPLScon conference in Virginia focused on the emerging use of MPLS to allow carriers to offer network-based VPN services.  Using MPLS a carrier can create IP-layer (Layer 3) tunnels through their network, thus providing isolation of customer traffic at a level equal to that of other tunneling mechanisms such as ATM and Frame Relay.

However, most of the excitement and interest at this conference was on yet another emerging use for MPLS dubbed "Layer 2 VPNs."  Layer 2 VPNs actually refer to two different technologies that allow Layer 2 traffic (such as ATM, Frame Relay, SONET/SDH or Ethernet) to be directly encapsulated within MPLS label switch paths (e.g. circuits).  At first this seems a bit peculiar, after all, isn't IP carried by Layer 2 mechanisms?  Doesn't this approach violate the OSI model we all know and love so much?

From a service provider perspective MPLS-based Layer 2 VPNs offer tremendous advantages.  Providers can use this technology to preserve existing services as they consolidate multiple Layer 2 and Layer 3 networks to a single unified network infrastructure.  For example, a provider can simply continue to offer Frame Relay or ATM services to their customers, even though the traffic may never actually be carried by Frame or ATM switches.

Start-up carriers such as Masergy are using Layer 2 emulation to provide Frame Relay and ATM services over their IP network, thus making their services more competitive for customers already familiar with existing Layer 2 services.

Two IETF working groups are currently developing standards in this area.  "Psuedowire Emulation End-to-End (PWE3)" is developing standards for encapsulation of Layer 2 frames and cells directly into MPLS.  This working group is the home of the "Martini Drafts" which are the most well known standards proposals for accomplishing this task.

Meanwhile, The IETF's "Provider Provisioned Virtual Private Networks (PPVPN)" working group is defining methods for provisioning these new and emerging VPN services.  From this group you'll find the also famous "Kompella" drafts which define methods for using BGP to provision Layer 2 VPNs.

From an enterprise perspective Layer 2 VPNs will most likely remain invisible.  Providers will simply continue to offer Layer 2 services as they always have.  However, the promise of a simplified network infrastructure may mean cost savings for enterprises in the long run.

For additional reading:
PPVPN Working Group:
PWE3 Working Group:
The MPLS Resource Center:

Irwin Lazar is a Senior Consultant for Burton Group where he focuses on strategic planning and network architecture for Fortune 500 enterprises as well as large service providers. He is the conference director for MPLScon and runs The MPLS Resource Center and The Information Technology Professional's Resource Center

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