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4 PCs are connected to a switch and this switch is aware of their MAC addresses (all 4 PCs). Computer A and Computer D have not communicated ever earlier hence they both don't have their ARP table updated with each other's MAC address.
Computer A wants to communicate with Computer D, hence it sends out an ARP request for Computer D's MAC address. This ARP would be a broadcast message (with FF:FF:FF:FF:FF:FF as destination MAC in ARP packet).
I'd want to understand as how switch will take it up as it's already aware of Computer D's MAC but Computer A is not. Will the switch let Computer D respond to the ARP messgage (the broadcast) or drop that ARP request and directly route Computer A's packet to Computer D (since switch is already aware of Computer D's MAC)?
2In addition to the answers, all operating systems that I'm aware of nowadays issue a Gratuitous ARP when the link is brought up. – AbraCadaver – 2016-08-08T15:35:43.737
@AbraCadaver That's not really relevant. Gratuitous ARP should only existing ARP cache entries, not create new ones. It's meant to force out obsolete entries when an IP moves or a NIC is replaced, not preload caches. – Barmar – 2016-08-12T17:29:10.070
@Barmar: No. Bridge/switch sees frame, records source MAC and port. That's how it works. – AbraCadaver – 2016-08-12T18:08:28.323
@AbraCadaver What does that have to do with how gratuitous ARP is processed? – Barmar – 2016-08-12T18:09:48.857
@Barmar: Gratuitous ARP is not processed by a switch. It sees an Ethernet frame with a source MAC come in on a particular port, that's it... – AbraCadaver – 2016-08-12T18:14:40.857
@AbraCadaver I never said it was. You're the one who brought up gratuitous ARP, and I was just pointing out that it doesn't have any effect on the machines that don't already know about each other (Computer A in the question). – Barmar – 2016-08-12T18:19:25.730