hey guys was wondering if someone can potentially explain this to me

but lets say switch-A just learned a Ethernet address on port 4 (00:0f:4b:02:63:ae) based on the forwarding table

now a station on port 6 with a Ethernet address of lets say (00:aa:bb:cc:dd:ee) sends to 00:0f:4b:02:63:ae, that is on port 4, what will that switch do with the frame and forwarding table?

does it just update the forwarding table? or does it perform some sort of algorithm and learn its new neighbors?

and how does it work exactly?

  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin
    Aug 11, 2017 at 2:53

1 Answer 1

  • the SRC MAC (00:aa:bb:cc:dd:ee) is checked in the forwarding table
    • add if not in the table
    • update (move) if it was on a different port
  • the DST MAC (00:0f:4b:02:63:ae) is checked in the forwarding table
    • not found will unicast flood to all other ports
    • otherwise forward on that port

In other words, the forwarding table is populated on receive with the source within the frame. If the destination isn't in the table, it'll go "everywhere".

(Note: some switches can have flooding disabled. In such a case, unknown destinations are dropped -- or sent to the switch CPU to deal with.)

  • I wasn't aware that you could disable the flooding operation. Is that feature present on most enterprise switches?
    – Ryan Foley
    Jan 15, 2015 at 17:45
  • 1
    Depends on the switch. Cisco 2960s: switchport block ? -- {uni|multi}cast Block unknown {uni|multi}cast addresses As one who deals with programming switch chips, every one I've seen supported it.
    – Ricky
    Jan 16, 2015 at 2:32

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