Why is the switch with the lowest mac address chosen as a root switch by STP?

[pmventura]

I know you can change the priority to force the root selection.  If newer and better switches have greater MAC address, why is STP still designed to choose the switch with lower mac address by default? 

[Edwin Summers · Accepted solution]

I'll defer to an elder historian as to why that particular field was chosen and why lower and not higher was preferred.  My guess would be something had to be chosen, the MAC was something that would be inherent in every device (not some extra feature that would need to be added), and IT people like lower numbers (we count starting at 0, right?).

But remember it's not necessarily newer=higher.  The first three three bytes are the OUI, and it appears that Xerox holds the market on the low numbers there.

But your question does emphasize the point why good design and implementation practices are necessary for optimal operation.  Component and topology state should often be "determined" rather than left to the ethergods to choose at whim.

[james-mitchell29]

According to "CCNP and CCIE Enterprise Core ENCOR 350_401 Official Cert Guide 2nd Edition" . . . .

..... 

Generally, older switches have a lower MAC address and are considered more preferable. Configuration changes can be made for optimizing placement of the root bridge in a Layer 2 topology to prevent the insertion of an older switch from becoming the new root bridge.

So basically preventing newer switches from preempting the older root switch.