Frame Forwarding by Layer 2 switch

My apologies if this is not the right forum to ask this, but I am a newbie when it comes to networking. I am currently going through the CISCO Networking Essentials course, and one of the Activities in the second module left me confused. Basically computers with MAC addresses 0E and 0F are both connected to a hub, which connects to a single Layer 2 switch port (Fa9). The question starts with the assumption that a MAC address table entry already exists for 0E (Fa9) but none yet for 0F. If computer with MAC 0E were to address a frame to computer with MAC 0F, how would that ever reach its destination? As I understand it, the switch, not "seeing" 0F in its table would flood the frame out all connected ports except Fa9 (incoming), but that means the frame will never exit via Fa9, so how is computer with MAC 0F ever supposed to receive the frame?

Thanks in advance for your help.

1 Answer 1


A hub simply repeats electric signals received on a one interface out of all other interfaces. So device 0F, as well as the switch, will receive the frame. In that case the switch will not forward the frame out the same port it received it on, but it doesn't matter since 0F already received it. Regarding learning of 0F - The switch will eventually learn 0F's address once 0F sends a frame.

  • 1
    Thanks so much! I suspected this would be the answer (the hub doing the work) but wasn't sure, but you have confirmed it for me. Really appreciate it.
    – Deepak
    Apr 26, 2022 at 4:15
  • 2
    "As hub floods traffic out of all ports Not exactly. A hub is a layer-1 device like a powered cable, so it repeats the signal out all other interfaces. It does not understand frames, so it does not flood the frames, it repeats the signals, valid or garbage, out all other interfaces.
    – Ron Maupin
    Apr 26, 2022 at 12:32

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