The purpose of the virtual stacking is to have one control plane for two switches, in case you have one switch down, other switch is ready to control. Make a note that both have its own data plane, so the straight answer is 2 switches in stacks= 1 control plane and 2 data plane (one switch is master and the other is slave, if one goes down, other will take over and make himself master) and the purpose is redundancy Important thing that the virtual stack will be anywhere in the network while physical stacking will be placed together https://meraki.cisco.com/blog/2012/05/virtual-stacking-the-missing-link-in-enterprise-switch-management/
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