MAC-adress "flapping" even between Vlan:s?

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May 1st, 2007
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In general, does a L2-switch has one MAC-adress-table per vlan or one common database?

How should a switch react if the same MAC-adress shows up on several ports at the same time, but on different Vlan:s? Is this "flapping" och just normal behaviour?

Best Regards

Jimmy Larsson, CCNP, CCSP

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Amit Singh Tue, 05/01/2007 - 08:26
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L2 switch have a common database i.e MAC TABLE and it learns the mac-address on each port, creates the table per port and vlan basis.

Switch> show mac address-table

Mac Address Table


Vlan Mac Address Type Ports

---- ----------- ---- -----

1 0030.9441.6327 DYNAMIC Gi0/4

Generally, A host connects to only one vlan and the address gets learned on a port in the same vlan.If somehow a host connects to a switchport using the Trunking on the NIC card, it will become a member of multiple vlans, the switch will treat as a normal behaviour.

-amit singh

wochanda Tue, 05/01/2007 - 09:57
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The switch doesn't mind if it learns the same MAC address on different VLANs, as it keeps each VLAN mac-address-table seperate.

If you see mac "flapping" messages, though, it is usually an indication of a spanning-tree loop, as it has to be learned on both interfaces multiple times in a short period of time for this to happen.

The best way to troubleshoot is to figure out where the MAC is "supposed" to be learned, then use the flapping messages trace the MAC address to whichever port it isn't supposed to be learned on but is, and figure out how its getting there. Usually it is a cable plugged mistakenly between VLANs.


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