Hi SJ,
Can u paste the config of the both ports??
may be :
u have made a trunk port as port-fast. And the most easiest way of detection of loop (before the network goes down) is those flapping message you are getting. It does seems that your network is having a L2 loop.
Disable the port-fast (on both sides) and then see if you are getting the flapping message.
If you are brave enough, you can also apply bpdu-guard to the port-fast (if there is a loop, it will change the port to err-disable / shut).
Normally you would enable portfast on trunks with spanning-tree portfast trunk since regular spanning-tree portfast
would only works on access ports.
Regards
