3 PCs (PC1, PC2, and PC3)connected through a Hub device

[ammaralabsee]

•  In a network of 3 PCs (PC1, PC2, and PC3)connected through a Hub device.If PC1 sends a unicast to PC2, which devices will receive the unicast? What if the device connecting all PCs together is Switch?

•  What will happen if the traffic is a multicast instead of a unicast?

[Richard Burts · Accepted solution]

When connected by hub if PC1 sends a frame then both PC2 and PC3 will receive the frame. It makes no difference whether it is unicast or multicast.

When connected by switch if PC1 sends a unicast frame to PC2 (assuming that the frame has the correct destination mac address) then it is forwarded to PC2 and PC3 does not see that frame. If the frame is multicast then both PC will receive the frame.

[Richard Burts]

I am glad that my explanation was helpful. Thank you for marking this question as solved. This will help other participants in the community to identify discussions which have helpful information. This community is an excellent place to ask questions and to learn about networking. I hope to see you continue to be active in the community.

[Joseph W. Doherty]

Just to expand a bit on what Rick posted.

"When connected by switch if PC1 sends a unicast frame to PC2 (assuming that the frame has the correct destination mac address) then it is forwarded to PC2 and PC3 does not see that frame."

Additionally, the switch would also need to "know" which switch port PC2 is connected to.  It does this by recording the source MAC(s) of frames entering each port.

If destination MAC port is unknown, the switch will send the destination frame to all ports (excluding the port it arrived on), on that switch, within the same broadcast domain.  This is very much like how the hub operates all the time.  On a switch, this is known as "unicast flooding".

"If the frame is multicast then both PC will receive the frame."

More advanced/featured switches offer "IGMP snooping".  I.e. they watch for IGMP packets indicating what multicast IP(s) a switch port "wants".  If they don't "see" a port wanting a multicast IP, they don't send that multicast to such a port.

So, to your question: "What will happen if the traffic is a multicast instead of a unicast?", if the switch is running IGMP snooping, ideally, multicast will only be sent to the PCs that want that particular multicast.  I.e. Neither PC might be sent particular multicast, one or the other might be sent it, or both PCs might be sent it.