vSwitches

vSwitches cannot be connected with any type of virtual uplink native to the hypervisor. Because of this, there are no potential loop conditions to avoid and thus vSwitches have no need to run Spanning Tree Protocol. This can be viewed as a benefit when comparing virtualized vSwitches to physical switches in your network infrastructure.

http://www.vmware.com/files/pdf/virtual_networking_concepts.pdf
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