Data Center Journal

Volume 27 | May 2013

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it CORNER Are network overlays the end of old school networking principles? By Marco Di Benedetto Network overlays are logical representations of networking paths built on top of one (or a set of interconnected) physical networks. In other words, network overlays define network virtualization. They are to network virtualization what virtual machines are to compute virtualization. Sprn O ne of the interesting effects of the software defined networking (SDN) movement is the spark in creativity around network overlay technologies. What are "network overlays?" Network overlays are logical representations of networking paths built on top of one (or a set of interconnected) physical networks. In other words, network overlays define network virtualization. They are to network virtualization what virtual machines are to compute virtualization. While this might sound cool and revolutionary, network overlays are not a new concept, and many different technologies have been built over the years to enable overlays with different properties. If you've used GRE tunnels, IPsec tunnels, or L2TP tunnels, you've already used overlays. Heck, even if you've used VLANs you've used overlays. Congratulations! But then, why such a tremendous focus on designing new overlay technologies for SDN? Why VXLAN, NVGRE and STT? One, probably unsatisfactory, answer is that most traditional overlay technologies are meant for the Internet or for telecom use, not for the Data Center. With network virtualization, we're probably for the first time finding a broad use case to run overlays inside a Data Center. Different requirements call for different technologies. When it comes to data center network overlay technologies, I like to broadly classify them into two classes: L2 overlays and L3 overlays. L2 overlays are overlay technologies that builds virtual L2 networks on top of a physical network. L2 overlays happen to be built on top of L3/L4 "underlays" (i.e. using L3 tunneling), and that leaves enough ambiguity for some marketers to get away with a term like "L3 overlays" to describe them. Unfortunately stretching the terminology in this manner can cause confusion between L2 overlays and a different set of technologies that are more appropriately defined as "L3 overlays." L3 overlays are technologies that overlay L3 segments on top of a physical network. Real L3 overlays dispense of L2 (i.e. Ethernet) altogether. That makes them more lightweight than L2 overlays: since L3 overlays don't deal with Ethernet, they don't need to emulate the Ethernet control and data planes. An L3 overlay virtualizes a point-to-point link between a pair of devices. An L2 overlay typically virtualizes a fully meshed any-to-any L2 network, and typically it attempts to emulate Ethernet, or at least its behavior. Personally, I don't expect L2 overlays to dramatically change a POD design or to introduce mainstream "POD-less" options. At the end of the day, L2 overlays define the boundaries of IP subnets, and cross-subnet connectivity can only be built leveraging L3 overlays. While some people are experimenting and trying to shift the balance between L2 and L3 using L2 overlays, I do believe that regardless of what technology you use to build subnets, keeping subnets C K G H Y K S F V T D S G P 28 | THE DATA CENTER JOURNAL www.datacenterjournal.com R E

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