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When the switch CAM table is empty and it receives an ARP message, does it add to the CAM table the source MAC address ?

In Packet Tracer I saw that it does add it, but I read in some book that the switch ignores them.

Thank you.

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up vote 6 down vote accepted

Switches don't know what ARP messages are and don't care about ARP. ARP is not a layer two protocol because it maps IP addresses and switches operate purely at layer 2 (Ethernet). As far as the switch is concerned, an ARP message is a packet like any other packet and they handle it the same way. If they didn't update the CAM table on unicast ARP messages, they wouldn't update the CAM table on any unicast message.

Switches learn MAC addresses from the source address of Ethernet frames on the ports, such as Address Resolution Protocol response packets. -- Wikipedia

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I'm not entirely certain ARP can be considered a L3 protocol. It is, in my opinion, not really well classifiable through the simplified OSI model. It has a well-defined place in the full 7 layer model, but in the 4 layer model it is a border-case between 2 and 3. By convention it is called a L3 protocol, but to the purist... – Tonny Jan 19 '13 at 23:16
@Tonny: I updated my answer to reflect the fact that some people (though I 100% disagree with them, maybe I'm a purist) do not consider ARP a pure L3 protocol. IMO, Ethernet is 100% complete without ARP and ARP is only needed to implement some other protocol on top of Ethernet, and that other protocol is IP. – David Schwartz Jan 20 '13 at 0:51
I understand your point very well. From a practical perspective in real life implementations it is L3. But when you look closely at the theoretical definitions of the OSI model... Believe me. I spend days discussing this when I was working on my CompSci Bsc. – Tonny Jan 20 '13 at 11:12

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