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Is it possible that there is one MAC address associated to more interfaces in the switch MAC table? If not, what is the behaviour when frame with known MAC comes from a different interface?

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When a switch receives a frame from an interface, it creates an entry in the mac-address table with the source mac and interface. If the source mac is known, it will update the table with the new interface.

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  • And by update you mean that the MAC will be removed from the previous interface? – Martin Heralecký Sep 9 '16 at 16:46
  • Yes, normally that is true. The switch will remove the MAC from the other interface. There is an exception, though: multicast L3 addresses are mapped to MAC addresses, which can be present on more than one interface. – waza-ari Sep 9 '16 at 17:25
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Is it possible that there is one MAC address associated to more interfaces in the switch MAC table?

No. A MAC address is an address of a unique NIC on the network, and each MAC address will only appear once in a switch's MAC address table. If a there are more than one NICs on the same LAN that share the same MAC address, it will cause all sorts of communication issues.

If not, what is the behavior when frame with known MAC comes from a different interface?

The Switch will not store the same MAC address on multiple ports. It will simply update its MAC address table with the location of the most recent frame arriving with the duplicate MAC address. If both hosts are transmitting constantly, that will cause the MAC address entry to bounce between the two switch ports (known as MAC flapping).

On the receiving side, the hosts will never get all the frames that are intended for them. Its like being in one conversation but only receiving every other word (and someone else receiving the remaining words).

Duplicate MAC addresses on the same L2 network causes both hosts to have communication issues. However, duplicate MAC addresses on different L2 networks (aka, separated by a router) will work just fine.

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Yes it is possible via MAC spoofing. If you are seeing the same MAC address on multiple ports then it is possible that someone is in your network doing some not nice things.

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Ive seen this before on some of the cisco 3750 switches I managed when I worked for one of the major airlines....in one of the IDfs the terminal would indicate a MAC which was a duplicate on multiple interfaces....this could be a few things, bad STP, interface issues, misconfiguration....it goes on. When you see it you get the old flappy flappy....its almost like a loop in a netowrk segment just not as disastrous...It will definitely cause your data traffic to flow incorrectly and give you unnecessary latency.

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