VPLS, a new Layer 2 MPLS VPN technology

but you must be aware of its limitations to decide if VPLS is appropriate for your network. David Passmore, Research Director of The Burton Group, a Network and Telecom Strategies Firm in Salt Lake City, Utah, expects the service to become extremely popular. Currently, only a few ISPs offer the service, but he predicts others to roll it out over the next year.

Network managers have become convinced of the advantages of Layer 3 MPLS over Frame Relay and ATM. VPLS offers the same advantages of reliability and QOS capabilities and adds the additional advantage of simplicity, but limitations on scalability and geographical distribution make it a poor choice for some networks.

A VPLS network appears to be a single shared LAN among all of your sites, making it a very simple network to understand and manage. Also, VPLS can carry non-IP traffic without any need for conversion or encapsulation. You interface to your ISP's network with an Ethernet switch. If Ethernet service isn't yet available at some of your sites, your ISP will supply a router that converts packets to the format required by the underlying media type.

On the other hand, L3 MPLS requires customer edge routers to exchange routing information with ISP routers using BGP. There should be no leakage of information between one ISP customer and another, but some customers are sensitive to this possibility and are hesitant to choose L3 MPLS for this reason. Some ISPs require customers to change their IP address structure to interface to an L3 MPLS service. According to Passmore, "L3 makes customers rethink their architecture. VPLS does not." Also, configuring and managing an Ethernet switch is easier than configuring and managing a router running BGP.

What are the downsides of VPLS? Like any shared LAN, all packets are delivered to every site, not just the actual destination. Ethernet VLANs enable you to confine traffic to a subset of sites, but all packets on a VLAN are delivered to each site included in the VLAN. You should choose VPLS only if you can segregate traffic so that you have no more than a few dozen sites in any VLAN. The actual limit will depend on the traffic level in heavily used VLANs. L3 MPLS can scale to much larger networks, since each site receives only traffic actually directed to the site.

A second limitation of VPLS is the fact that the standards are still evolving. As a result, a VPLS network cannot be split between multiple Autonomous Systems. This means that the same ISP must offer service to all of your sites. According to Internet Engineering Task Force member Oded Bergman of MRV Communications in Chatsworth, California, current standards work is concentrating on the details of VPLS operations within an Autonomous System. The standards committees will not begin work on issues for connecting across Autonomous Systems until current work is complete. This means that if one ISP does not serve all of your sites, you will not be able to choose VPLS in the near future.

Cost is always an issue. Some ISPs will charge based on number of sites; some by traffic level. You need to investigate pricing carefully with your ISP.

Overall, VPLS offers an attractive choice, but you must insure that your network does not exceed its scalability limits and, for now, that one ISP can serve all of your sites.

David B. Jacobs has more than twenty years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software start-ups.