Layer 2 address learning

November 2, 2006 at 14:57:19
Specs: any, any
I understand how a LAN switch learns the destination and source MAC address of two terminals communicating for the "first time" and how subsequent communications between these two terminals would not require a second broadcast message to find the location of either recipient.

My question is, is that also true for other hosts who have not yet communicated with the original two communicating hosts? If the switch now knows the port assignments for hosts A and B, I can't imagine why a communication to either A or B from any other host on the network would require a broadcast message from the switch.

Seems like a dumb question as I am asking it, but I just want to make certain. Thanks!

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November 2, 2006 at 15:17:10
I think you should get ethereal and use it on test systems. Use a hub or switch and monitor the connection and view it. Boot one system and view it's data. Leave the systems on with not user input and see what you get.

I read it wrong and answer it wrong too. So get off my case you goober.

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November 2, 2006 at 16:26:51
Unfortunately, I don't have a system to test it on. I teach a class in data com, and it is all pretty conceptual. Not much hands on. I was just trying to pin down this one concept for certain.

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November 2, 2006 at 16:56:50
It isn't a matter of other hosts communicating with A or B. The switch doesn't care about that. It builds a table of mac address to port number based on when a computers network card first makes a live connection to the switch.

There are no broadcasts, at this layer, which is where I think your question lays. Broadcasts don't come into play until layer 3 and ip.

I guess you could say a hub broadcasts but that isn't quite right. A hub repeats to all ports. IP does broadcasts.

I hope this helps.

Give a person a fish, they eat for a day. Suggest they internet search and they learn a skill for a lifetime.

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November 3, 2006 at 17:32:15
I would have to disagree with your last statement.

An ARP broadcast is an IP to MAC Address resolution protocol which occurs at the Data Link Layer of OSI. RARP is also a Data Link Layer broadcast.

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