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i am doing a research about some switchs behavior in some different situations.

And, i got a situation that i really need to clarify. So, that's the point:

We all know that when a machine wants to send a packet to another one in the network, it sends a ARP Request packet to discover the MAC Address from the Destination machine, when the Destination Machine answers with a ARP Reply packet, the switch records that information on its Table and now it knows that HOST1 is on Port1 and HOST2 is on Port2, for example.

So, that's OK, but, what's happen when HOST1 has a static MAC from HOST2 on its O.S. and in the other hand, HOST2 has a static MAC from HOST1 as well? So, there is no ARP traffic on network because both hosts already know their destination machines' MAC Addresses.

I made this test, and looking deep on captures, i really don't see ARP Traffic, as expected. But, i see that ICMP traffic is correctly forward to the destination machines, so, there is no Broadcast. Even with Switch's table and ARP Cache without any information about other hosts on the network.

So guys, how this process really work? How switch can forward correctly to the destination port if it doesn't have that port information on its MAC Table?

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up vote 1 down vote accepted

Switches operate at L2. They treat ARP traffic just like any other traffic. A swtich will learn from any unicast traffic.

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Yeah, but that's not the point. The question is: how it will learn how to send to the correct port with a unicast traffic? Is it necessary to send an ARP Request to discover in which port the machine is?? The point is that i can't see broadcast traffic on my captures, and Switch MAC Table is null before the communication, and after that, it shows MAC from HOST1 and HOST2, even without broadcasting it. – StarkBR Jun 11 '13 at 3:16
I don't understand what your confusion is. There's nothing special about ARP as far as the switch is concerned. Whether there's ARP or isn't has no effect on the switch whatsoever. It can discover the port based on any unicast traffic. It sees which port it received the packet on and it sees the source MAC address. That's all it needs to populate its CAM table. – David Schwartz Jun 11 '13 at 3:38
Ok. Let me summarize the question: How switch work when it receive a unicast traffic with a Destination MAC that it doesn't have on its CAM? That's the point... – StarkBR Jun 11 '13 at 12:39
Does switch create a new ARP Request to discover which port destination MAC is? Or it will flood all ports with that unicast traffic (except port that it orgininally comes) ?? – StarkBR Jun 11 '13 at 12:47
Switches are (theoretically) pure L2 devices, they have no idea what an ARP request is. They can't create one. If a switch receives unicast traffic to a destination MAC not in its CAM table, it floods the traffic out all ports except the originating port. – David Schwartz Jun 11 '13 at 18:08

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