A virtual switch is a software program that allows one virtual machine (VM) to communicate with another.
Just like its counterpart, the physical Ethernet switch, a virtual switch does more than just forward data packets. It can intelligently direct communication on the network by inspecting packets before passing them on. Some vendors embed virtual switches right into their virtualization software, but a virtual switch can also be included in a server's hardware as part of its firmware.
One of the key challenges with server virtualization has been to figure out a way to allow network administrators to move VMs across physical hosts without having to stop and reconfigure them individually. Because moving VMs across physical hosts in a scalable way is time consuming and can potentially expose the network to security breaches if not done right, these concerns may prevent an enterprise from taking its virtualization initiative beyond simple server consolidation to more dynamic resource allocation. That's where advancements in virtual switches can help. Because a virtual switch is intelligent, it can potentially be used to ensure the integrity of a VM's profile -- including its network and security settings -- as the virtual machine (VM) is migrated across physical hosts on the network.
See also: virtual systems management
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Chris Wolf takes a look at "Virtual switch security: VMware, Virtual Server and XenExpress."
Shamus McGillicuddy explains how "Virtual network switches add scalability to server virtualization."