Hello Paul
That’s a very interesting experiment you performed, and it’s very useful for all of our readers. So it seems that when you issue the shutdown command, the router sends a resign packet before it actually goes down. This causes the standby router to assume the active role. This behavior is not standardized, and you may see slightly different behaviors in different model devices.
Now if instead of shutting down the interface you simply unplugged the port, you would not see a resign packet, and you would likely need to have the hold time expire before the standby router becomes active. Such an experiment would be more appropriate with real devices, I don’t know how a device in CML would react to the disconnection of a link like that. If you do try it out let us know.
I hope this has been helpful!
Laz