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I could go on forever about Virtual Switches used by Vmware, however, here’s a bit of a rundown.

Virtual switches fulfill similar functions to physical switches in an infrastructure ie. they grant virtual operating systems access to the physical network.

That’s not ENTIRELY true.

Virtual switches are also used for the Service Console (or management IP address) for the ESX server itself, They are also used for IP storage and Vmotion capabilities. More on these later. For the moment, I am just talking about the capabilities for virtual switches for virtual machines and generally about the switches themselves.

The switches used for virtual machines are known as Virtual Machine Port groups. They are unmanaged switches in that I cannot specifically connect my webserver to the first port in the virtual switch. At the moment! With the release of Vmware 4, it’ll be interesting to see what the partnership between Cisco and Vmware produces.

Virtual switches are, as of writing this, a single-tier topology. Each virtual switch has it own forwarding table. However, these tables are only locally significant, so configurations cannot be passed from one virtual switch to another even if the entries in one switch’s table exist in another virtual switch.

So, how are they similar to physical switches? They have a MAC:port forwarding table, do lookups on frames to figure out destination MAC addresses. Forward frames to one or more ports for transmission. They support standard VLAN segmentation ie. they are dot1q compliant; no ISL or LANE etc.

How are they different from physical switches? No spanning tree protocol, network traffic cannot flow directly from one virtual switch to another. This means that traffic from a virtual machine plugged into a virtual switch has to leave the ESX server to be passed to another virtual machines plugged into another separate virtual switch. Also, there is no trunking between virtual switches.

Or, to put it another way, there is no way to directly "cable" one virtual switch to another. All this, hopefully, makes sense below.

The other similarity between the virtual and physical switches is CDP capabilities. Virtual switches, like most vendor’s physical switches, are CDP aware. Via the command line I can enable or change CDP behaviour on the virtual switch. The four potential CPD modes are down (CDP disabled), listen (the default, detects and displays information about other CDP devices), advertise (broadcast but do not detect or display information), both (both advertise and detect and display CDP information).