1

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)

Diagram..

  • What are port 1 of the 4948 and port 3 of the 3650 connected to? There must be something in common, directly or indirectly. What are the configurations of the switches? You probably have a layer-2 loop, and you need to determine what is causing it. – Ron Maupin Apr 17 '16 at 15:22
  • 1
    These are connected to different layer 2 links that go to remote locations. The layer 2 data links are provided by an ISP and we have already opened up a ticket with them to see what might be happening. In the meantime, I am interested in knowing why one switch isn't showing the MAC flapping messages, since I wouldn't want something like this to happen again and not have any warning about it. – Anne-of-GG Apr 17 '16 at 15:37
  • 1
    Probably because the flapping is happening much faster on one switch than the other. A switch will allow some switching of a MAC address from one port to another because you may unplug something like a laptop from one port and plug it into another. There is some "give" for that situation, but when it happens regularly and frequently, a switch will complain. You exceed the threshold for the 3650, but not for the 4948. That is a clue you can use to troubleshoot the problem. – Ron Maupin Apr 17 '16 at 15:41
  • This is another reason I really hate extending layer-2 over a WAN. We live in a layer-3 world, and there is really no reason to extend layer-2 like this. The latency exacerbates any layer-2 problems you may have, and routing the traffic will prevent this sort of thing. – Ron Maupin Apr 17 '16 at 15:43
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 8 '17 at 3:18
1

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.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.