I met quite often the opinion that Cisco's implementation on Ethernet loopback keepalives (Ether type 0x9000) make life difficult when your STP doesn't converge fast enouch. We had such a bad experience when all our switches put their uplinks in Err-disable status after one port was wrong connected and STP didn't converege before "looopback" was detected.
How exactly does the loopback keepalive work? I know that it sends the Ethernet frame out of a port with DEST and SRC MAC address of that interface. According to normal logic the upstream switch will forward that ethernet frame back to the original interface and the first switch will get their own frame back.
But in which case the loopback condition will be detected?