Hey guys well i kind of understand what switches do with the forwarding tables.

example: Switch-A has just learned a Ethernet address of 00:0f:4b:02:63:ae is on port 4 based on forwarding table. Now, a station on port 6 with an ethernet address of lets say 00:aa:bb:cc:dd:ee sends to 00:0f:4b:02:63:ae

answer: I already know, 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

but now lets say same switch-A now receives a frame on port5 with an ethernet address of of 00:41:51:61:71:81 and a destination Ethernet address of 00:a2:b2:11:22:33.

now how will this switch update the forwarding table and what exactly would it do with this "frame it is receiving on port 5" will the switch do the same thing, but how ill it handle the destination

  • This is the exact same question you asked yesterday (16151)
    – Ricky
    Jan 16 '15 at 4:04
  • Every frame on every port goes through the exact same logic I explained yesterday. What on earth would make you think a different mac or port would do anything different?
    – Ricky
    Jan 16 '15 at 4:30
  • because the mac is not on the table this time. its not connected. so that mac is outside of the switch
    – Will Chap
    Jan 16 '15 at 21:13
  • Not in table === unicast flood. It doesn't care if the destination exists; it's job is done on transmit.
    – Ricky
    Jan 16 '15 at 22:46

It will search its table for 00:a2:b2:11:22:33. If the table hasn't been populated with that address because a frame with that address as a source hasn't been seen, the frame will be flooded.

The switches uses source addresses to build the forwarding table, but searches for destination addresses when it needs to forward a frame.

If the switch is making a decision based upon the source, then you're talking about filtering instead of forwarding and the decision is typically whether to continue forwarding or to drop (e.g. port security).

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