Many people have told me that spanning-tree (STP) is not needed or that it only causes problems. Some say that now that Cisco VSS (Virtual Switching System) is available, the need for STP has been eliminated. Most of the misconceptions around STP come from the lack of understanding. Not using STP on your network is like driving your car without insurance. Sure you don't need insurance for your car to operate, but the moment you get in an accident, you wish you had it.
Spanning-tree protocol was developed to automatically block traffic from going over a redundantly connected links between switches. It was a requirement to implement a redundant core or distribution layer and dual home connections between the switches. Without STP, the second link connected to another switch would cause a spanning-tree loop and take down your network. What if you don't want redundant links or to run STP? Are you willing to take the chance that no one will accidently connect a redundant cable in your network? Even with VSS, other non-VSS enable switch could be connected causing STP loops.
There are many methods to prevent STP loops including PVST+ Root Guard, Loop Guard, BPDU Guard or the use of Rapid STP which includes these features in an industry standard. They cause no negative impact to your Cisco Switches and insure you against STP loops crashing your network.
I recommend using Rapid Spanning-Tree (RSTP) in all switches that supports it. It combines many Cisco PVST+ features and is an industry standard as well as backwards compatible with PVST+.
Do you have any STP horror stories or experiences you want to share?