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Feature StoryBy Alex FosterThe Return of Layer 2How Data Center Networking Requirements Will Change the WANFor the past five years MPLS-based IP VPNs havebeen the dominant WAN transport solution fororganizations of all sizes. Originally adopted anddeployed by large organizations to support interconnectionbetween thousands of sites, IP VPNs quickly trickled downmarket to the point of ubiquity. The widespread adoptionof IP VPN has taken place because it offered a complete solutionto a common corporate need – secure, fully-meshedIP connectivity with quality of service between locations.Cavalier’s Alex FosterWhile WAN architectures have converged on a Layer 3 approach,a quiet revolution has been under way in data center networksand is increasingly creating requirements that IP VPNs cannotpractically meet. Modern data center networks are designedaround the requirements of virtualization and SAN replication.One of the biggest requirements for fully leveraging virtualizationis that virtual machines be able to move between physical hostswhile maintaining their IP addressing. Virtual machine movementwithin a data center is already common and has driven areinvention of data center switches and LAN architecture. Virtualmachine mobility between data centers for disaster recovery isthe next frontier, and the networking requirements around thistechnology and SAN replication will drive a new set of connectivityrequirements not easily met by IP VPNs.Thus far vendors and network architects have responded withseveral strategies for accommodating these needs within theexisting WAN architecture. The well-tested solution is simplyto operate outside of the IP VPN and deploy SONET orWDM connectivity between the data centers to act as a LANextension. This solution is costly, does not scale with severaldata centers, and it may be difficult to even get connectivitybetween data centers if they are served by different carriers.A more recent solution to extending Ethernet between datacenters is to overlay Ethernet traffic on top of IP VPN networksusing tunneling to encapsulate Ethernet frames intoIP packets and then strip them back out at the far end. Thissolution eliminates the operational expense of running parallelnetworks but requires a substantial hardware investment and asignificant amount of expertise to configure and maintain.Both solutions are far from ideal. The preferred solution formost mid-size businesses increasingly will be carrier-deliveredLayer 2 VPNs. This category includes services such as virtualprivate LAN service, E-LAN and transparent LAN services.These services are delivered over the service provider’s MPLSbackbone but leverage functionality in the provider’s edge todeliver a Layer 2 service based on virtual bridging rather thana Layer 3 service based on virtual route forwarding. This givesthe customer a network that behaves like a wide area Ethernetswitch. For many companies this single Ethernet WAN can serveas a solution to interconnect both data centers and offices.VPLS allows customers to solve many of the common data centerinterconnection problems in a much simpler and more intuitiveway than in an IP VPN environment, and is far more affordablethan SONET/WDM solutions. Without having to peer with serviceprovider routers at Layer 3, subnets and Layer 2 domains caneasily be extended between data centers. With complete control ofthe IP layer, routing can be based on the preferred protocol ratherthan a limited set of protocols supported by the service provider.Perhaps most importantly, organizations can tune their routingprotocols to deliver convergence times far quicker than possiblein an IP VPN environment. Finally, many technologies that havea long history of deployment in data center environments suchas hot standby router protocol can be leveraged in the WAN forfailover between data centers with complete IP mobility.Although there is concern in some corners about latency overthe MPLS core, these concerns are generally unwarranted. Thelatency imposed by MPLS hops in the carrier core is on theorder of 12 to 50 microseconds, nearing irrelevance even in aworld of single-digit millisecond requirements.While VPLS and E-LAN solutions are not yet common, theyare growing rapidly. Virtually every major service provider hasannounced some form of VPLS offering, and customers arequickly realizing that removing the service provider from the IPlayer of their network can greatly simplify their lives and speedthe deployment of next-generation data center solutions. Theinstances where separate corporate and data center networks arejustified will continue to decline, and the likely winner in thesecases will be Layer 2 VPN solutions such as VPLS. ITAlex Foster is product manager, data and managed services atCavalier Telephone (www.cavtel.com).GoTo:58 INTERNET TELEPHONY ® February 2011 Table of Contents • Ad IndexSubscribe FREE online at www.itmag.comGoTo: