Hello David
Hmm… that doesn’t seem to be the way the Max Age timer works. I think you’re probably referring to the Message Age, which is a field found within the BPDU header. This value actually starts out at 0 when the BPDU is sent by the root bridge, and it is incremented by 1 every time it is received and relayed by another switch.
When a switch receives a BPDU, it checks the BPDU’s Message Age and compares it with its own Max Age timer. If the BPDU is considered fresh (i.e., the Message Age is less than the Max Age), the switch processes the BPDU and resets its own Max Age timer for that port. If the BPDU Message Age exceeds the Max Age, the BPDU is discarded, and the switch might start considering changes in topology since it hasn’t received a valid BPDU for a while.
The Message Age value actually works more like TTL does for IP. It prevents BPDUs from looping throughout the topology.
Now this begs the question, if you have an STP topology that has some switches more than 20 “links” away, then the Message Age will always be larger than the Max Age for any switches that are that far from the root bridge.
Well it is best practice to keep your Layer 2 topologies much smaller, and you should never encounter such a large L2 STP topology. However, if you do, you would have to adjust the Max Age timer appropriately in order to allow for such large L2 infrastructure.
Why were these numbers chosen? Because they work. Even though the message age is not really in seconds, but it measures hops, it is compared to the Max Age which is in seconds. It may sound strange, but the design has been tested, and these values and these mechanisms work, for topologies that are of reasonable sizes. Does that make sense?
I hope this has been helpful!
Laz