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Virtualizing the Network

One of the reasons it’s a good idea to make the change to network virtualization is that it allows network admins and users to fully realize many of the awesome features of server virtualization, such as vMotion, snapshot backups, and push button disaster recovery (to name just a few). Indeed, the most common reason for virtualizing the network is precisely to get VM mobility and vMotion to work.

In Chapter 9, “Multitenancy and the Problems of Communal Living,” you were introduced to VXLAN, which is VLAN technology with some extensions that allow it to tunnel Layer 2 frames through the IP transport network, as well as extend the number of VLANs beyond 4096. The “tunnel” that this creates allows it to bridge virtual extensible VXLAN tunnel endpoint (VTEP) devices across a network, making data transfers easy and simple regardless of where the endpoints reside (or if they move).

As noted earlier, the VMs supporting applications or services require network connectivity via physical switching and routing to be able to connect to other VMs switching the data center or cloud and with clients of the data center over a WAN link or the Internet. In addition, in a data center environment, the network also requires security and load balancing. The first switch encountered by traffic leaving the VM is the virtual switch (hypervisor), and from there a physical switch that is either top of rack (TOR) or end of row (EOR). In other words, once traffic leaves the hypervisor, it is on the physical network, and unfortunately that network cannot easily keep up with the rapidly shifting state of the VMs that are connected to it.

The way around this issue is to create a logical network of VMs that spans the physical networks the traffic travels across. VXLAN (see Figure 10-2) does this just as most network virtualization does—through the use of encapsulation. Unlike simple VLANs, though, which are limited to 4096 of these logical networks on any given physical network, VXLAN can create about 16 million. That scale is important when it comes to large data centers and clouds.

Figure 10-2

Figure 10-2 VXLAN allows for millions of logical partitions across a physical network.

Imagine that you have two VM clusters on a network, and imagine that a router separates those clusters because they are in different data centers. Both clusters in this case are on different VLANs. For these two VMs to talk to each other, the traffic between them must be routed. Now suppose you want these clusters to be on the same VLAN.

As shown in Figure 10-3, by using VXLAN, you can set up a VTEP that encapsulates or wraps the VM traffic on one end for transport over the routed network and then decapsulates (strips off the wrapper) on the other end. This effectively creates a logical network between the two clusters, which now appear to be on the same switched segment of a local network.

Figure 10-3

Figure 10-3 VTEPs create a logical network between the two clusters, which then appear to be on the same switched segment of a local network.

So, what’s the big deal?

If you are new to networking, this might not seem like such a big breakthrough. If you are familiar with networking, though, you might be thinking, “This is just another way to create VLANs.” There is more to it than that, though, because network virtualization in general, and VXLAN in particular, has some key benefits that become important at data center/cloud scale:

  • First, this ability enables migration to a software-defined data center model. Using a vSphere administrator to provision VMs that can communicate with each other over different networks without having to involve the network team to configure the physical switches and routers eliminates one of the biggest chokepoints in the flexibility that data center virtualization affords us.
  • This technology smashes through the previous limitation of 4096 VLANs
  • VXLAN runs over standard switching hardware, and requires no need for software upgrades or special code versions on the switches. Therefore, you can virtualize your network using the stuff you already have.

In summary, network virtualization, although an older technology, plays a key role in the creation of virtualized data centers and clouds. It is also one of the key drivers that allows and enhances both NFV and SDN, as you will see in later chapters.

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