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I'm learning the network basics at my university this semester. I had a question on the mid-term exam and unfortunately ,I answered it wrong. I have a final exam about an hour from now, and I still don't know what the correct answer of that question is. However, here is my question:

A network of several PCs, switches, and one router. PC 1 has a MAC address X. PC 1 usually sends packets to PC 5 and these packets pass through Switch Z. Inspecting the MAC address table of Switch Z for a very long period revealed that the PC 5 MAC address never shown in the table. Is there anything wrong with the switch? If yes, show what is wrong with switch, or otherwise, explain why such this is happening.

and here is my wrong answer:

Maybe because PC5 is recently connected to the network, hence it cannot find the MAC address in the table. So, it should send a broadcast to the network for gaining the MAC address of PC5.

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2 Answers 2

up vote 6 down vote accepted

Normal L2 switches would never actively ask the PCs on the network what their MAC addresses are. So the only way for the switch to know if to passively pickup traffic from the PC. If PC5 never sends anything, which the Question doesn't mention PC5 ever sending, then the switch's MAC table would never know what PC5's MAC is.

Also, the table entries age out over time. PC5 might have been in the table a while ago and simply aged out due to inactivity. Connecting recently or a long time ago has nothing to do with this.

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MAC's are stored when data is sent to the switch. PC1 send message to switch, which then learns PC1's MAC, but since the switch doesnt know the MAC for PC5, then it sends a broadcast out all ports, to which only PC5 will reply, thus now adding the MAC for PC5 to the switch.

So, no, nothing is wrong with the switch.

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