One part of the explanation of the different behavior between Ethernet/FastEthernet and serial interfaces is that the serial interfaces use a keepalive mechanism that sends keepalive packets over the serial interface and must receive keepalive packets over the serial interface. If the correct keepalives are not received then the serial interface shows up and down. The keepalive on the Ethernet/FastEthernet does not work quite the same way and so the behavior is different.
Another part of the explanation is that while the serial keepalive works end to end over the link the Ethernet/FastEthernet does not check end to end (from router to router or whatever is connected on the Ethernet). The Ethernet/FastEthernet only checks to the next connected device which is usually a switch. So your router Ethernet/FastEthernet will report up and up because it is connected to a switch which is working and does not detect that the router on the other end of the link is down.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...