​​​Here are the basic rules of STP


1. Lowest bridge ID (Priority:MAC Address) switch becomes the Root-Bridge
2. Each non-root bridge should have ONE root port (RP) which is the port having lowest path-cost to Root Bridge.

If the path cost equal, then here are the criteria:

  • 1. Lowest Sending Bridge ID
  • 2. Lowest Port Priority (of sender)
  • 3. Lowest Interface number (of sender)​

3. All ports in Root Bridge become Designated Ports (DP)
4. Each segment should have one Designated Port (DP)
5. All RP/DPs will be in FORWARDING state & all other ports will be in BLOCKING state.

You may wish to look at this animation:

You can see some detail here also:


By vichhaiy Posted in Cisco

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