Spanning Tree Protocol Basic

I have following questions on STP.
**1) Why STP selects lowest MAC Address?** Can you provide an explanation.


Thanks in advance!

-Best Regards!

Hello Rebecca

The root bridge is chosen based on the configured bridge priority. If the bridge priority is the same between several routers, there must be another method by which the root bridge is to be chosen. Because MAC addresses are unique, they can always be used as a “tie-breaker” in these cases. The fact that designers chose the LOWEST MAC address as the root bridge for STP is just an arbitrary method of choosing a single root bridge. They could have designed it as the highest MAC address as well. The results would have been the same.

So it is simply by definition that the lowest MAC address is chosen as a tie breaker to choose the root bridge for an instance of STP.

I hope this has been helpful!


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Thanks alot! Much Appreciated.

Actually, there is a reason Cisco chose the lowest MAC address as opposed to the highest. It has to do with the how MAC addresses are assigned to manufactured equipment. The newer the equipment, the higher the MAC address. Cisco chose the lowest MAC being the tie-breaker to save customers from themselves when not properly implementing STP. If you simply pull switches out of the box and turn them on, then the MAC becomes the deciding factor as to your root bridge. If Cisco had chosen the highest MAC as the tie breaker, then every time, in this scenario, you put a new switch in your environment, the STP topology would change such that the new switch would be the root bridge.

So, think of Cisco choosing the lowest MAC address as a way of keeping customers’ STP topology stable!

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Hello Andrew

Thanks for that. :slight_smile: You learn something every day!


Thats Interesting! thanks Andrew!