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I've read different sources of information, and the only answer I found is that the source mac is needed to fill the switch's table (the one that maps the address to the port).

Is it right? What happens if the two computers are directly connected? There is no use for the mac address in this situation?

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    It depends on how they're "directly connected"! Serial cable? Cat5 crossover? Bluetooth? The answer could be different depending on what your back-to-back connection looks like. If the connection uses Ethernet, then a MAC is absolutely required. – Keller G Feb 8 '15 at 18:27
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  1. You're correct about switches learning device location (port) via the source MAC address. That alone means they're necessary everywhere.

  2. In your example with two computers/servers connected they're still required because those computers/servers could have multiple interfaces. MAC addresses are per interface, each interface would have to be able to discern what traffic is destined for it. The network stack of your OS doesn't care what it's connected to, it's going to interoperate the same with anything.

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    Even if there is only one NIC in both computers, the PC looks for a mac address so it knows which frame is addressed to it. The PC has no idea whether it's connected to a switch or directly to another computer. – This Feb 8 '15 at 15:36
  • Uh, @MikePennington did you mean hub? A hub rebrodcasts all packets on all ports, A switch is more discrete. But indeed your point is right on, and more so. – hildred Feb 8 '15 at 17:21
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When 2 Ethernet equipments are back to back connected, with the same cable to the 2 ports, the MAC layer is in situation of connected mode.

The native behavior of the MAC layer is connectionless mode, with source address and destination address.

A connectionless mode protocol can operate in connected mode, but a connected mode protocol (as ATM, MPLS, PPP) can never operate in connectionless mode.

So you're right, in this case, the Ethernet is simply used in connection mode.

The behaviour is same with PBB-TE carrier Ethernet.

Best regards, Michel

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