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Spanning Tree Protocol

Michael Reeve
Michael Reeve used Ask the Experts™
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I have heard that Spanning Tree Protocol selects by default the oldest switch by manufacturers date for the Root Bridge. This is based on the lowest Bridge ID (combination of priority and MAC Address) placed on switches, since new switches will have a higher numbered MAC Address (I realize you can set the priority to change the root bridge). My question is, if the priority is the same on all switches (for example), will STP select the oldest switch strictly do to the fact it will have the lowest MAC Address and thus the lower Bridge ID? Does anyone have credible sources to make this claim or is this urban legend? Thanks very much in advance.
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Well its going to be hard to find a company line stating this exact truth but it is the truth. It just goes without saying that as more devices are created, the numbers go up. Here is link to the Cisco Learning Network where a CCNP states this to be true.

https://learningnetwork.cisco.com/thread/21954
Don JohnstonInstructor
Top Expert 2015

Commented:
My question is, if the priority is the same on all switches (for example), will STP select the oldest switch strictly do to the fact it will have the lowest MAC Address and thus the lower Bridge ID?

This is correct. It's in the 802.1d spec. The age of the switch, however, is coincidental.  If a manufacture would assign MAC addresses to their switches starting with the last usable address in their OUI, then the opposite would be the case.

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Commented:
Exactly what I was looking for. Thank you!