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Same Mac Address learnt in two different port accessing two different VLAN

  • 31 January 2019
  • 3 replies

What happen if I receive the same Mac Address in two different port of a single switch. Of course the ports of the switch are in two different VLAN. The switch is Extreme X460 G2 (or X440 G2). I have never used an Extreme switch and I still did not receive one to make a test.

I understood that if the switch create different FDB (or Mac Address table) for each VLANs, it should work fine.
Is this creating a conflict of Mac Addresses or the extreme switch will manage creating as many FDB as VLANs are.


3 replies

Userlevel 5
Hi Formia,

Sounds odd but AFAIK each VLAN has its own FDB (it happens automatically) so they shouldn't interfere with each other.
Just asking, what is the use case?

Kind regards,
Userlevel 6
Single mac on various vlans is normal.. Since each vlan is it's own broadcast domain and the table for each vlan forwards traffic only in that vlan on the ports configured with the vlan it will not be a problem..Here is such an example.. one core router for sip gateway with multiple layer 2 vlans provisioned ... Router has only one mac and yet the switches never get confused about where to send the traffic and the traffic stays only in the vlan provisioned.. No cross talk...

BD8810_CYGA_2.5 # sh fdb 00:22:83:d5:8f:cb
Mac Vlan Age Flags Port / Virtual Port List
00:22:83:d5:8f:cb plw_p2p_ipglobal_woffice_1302(1302) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_ipswich_1307(1307) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_vman_ipglobal_swfwy_1243(1243) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_westheimer_2647(2647) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_wickchest_2110(2110) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_postoak_1406(1406) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_spaceland_545(0545) 0000 dhm 9:6
00:22:83:d5:8f:cb plw_p2p_ipglobal_gessner_1411(1411) 0000 dhm 9:6
Thank you guys !!

We use kind of PLC providing safety system that assign mac address on the ports based on the slot and node installed. So there can be a chance to have the same mac address.