Lessons · Network switches · flooding the unknown
What it does when it has not learned yet
A frame for an address the switch has never heard is sent out of every port in that VLAN except the one it arrived on.
Hone is a place to practise programming. This is one of its lessons, written out in full and free to read without an account.
What it is for
It explains why a brand new device works instantly, why an empty MAC table is not a fault, and why a loop is so destructive: flooded frames go round for ever.
How to think about it
Unknown means flood, known means forward to one port, broadcast always floods. Three rules, and they explain almost every 'why did that traffic go there' question.
Worked example
enableThe # prompt.
show mac address-tableEmpty. Every frame arriving now goes to an unknown destination, so every one of them is flooded within its VLAN.
show vlan briefWhich ports would receive that flood: the ones in the same VLAN, and no others. The VLAN is the boundary.
show interfaces statusAnd of those, only the connected ones actually carry it anywhere.
Your turn
Show the table that decides whether a frame is flooded or forwarded.
show address-table
Console into a switch
The trap
Reading flooding as a fault. It is normal and it is brief: the reply teaches the switch the address, and the next frame goes to one port only.