for tunneling Mac packets, ethernet switches need source mac address? if yes or no, why?


  • You'll need to provide more information. Are you referring to Q-in-Q tunneling? Or just passing the Ethernet frame through a switch? Nov 24, 2016 at 7:03
  • just passing the Ethernet frame through a switch Nov 24, 2016 at 7:09

1 Answer 1


Ethernet switches use the source MAC address of an Ethernet frame to populate their forwarding table.

When a switch is powered up for the first time, it's forwarding table is empty.

When it receives a frame sourced from MAC A on Port 1 destined for MAC B, it will not know where to send it, so will flood it to all ports except port 1 (assuming single VLAN across all ports).

It will then update the forwarding table and map MAC A to Port 1.

Now, whenever a frame is received destined to MAC A, it will be simply sent out Port 1 and not all other interfaces.

As more traffic is sent through the switch, the forwarding table will start to be populated such that all hosts in the LAN will become known and less and less flooding will occur.


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