I have two Cisco switches in the same VLAN, physically connected to each other. One of the switches is a 3650, and the other is a 4948. On the 3650, I can see MAC flapping messages between 2 ports, one of which is the port that has the cable that goes to the other switch. On the 4948, I don't see any MAC flapping messages despite the fact that I have seen the same MAC address on the port that goes to the 3650, and on another port on this switch.
Why is this happening and how can I fix it so that I also see MAC flapping messages on the 4948?
I have the following logging configuration:
- logging buffered informational
- logging buffered 64000
- logging event link-status global
- logging event link-status (configured under each physical interface, since the global configuration didn't seem to be working)
Best Answer
You probably don't have MAC flapping on the 4948. MAC flapping is where traffic from a MAC address is originated on more than one port in a short period of time. It seems to be happening on the 3650, but not happening on the 4948. This is specific to the switch, and the error will not be sent to other network devices.
That it is happening on one switch, but not the other, is a clue to help you troubleshoot where the problem may lie. This is probably caused by a layer-2 loop. You should investigate how the switches are connected, both directly and indirectly. There may be an STP misconfiguration on how the ports showing the MAC address are connected. This may be caused by someone plugging in a hub or dumb switch into multiple ports. Unfortunately, you may need physically check things out. Using portfast with BPDU guard on access ports may prevent this situation.