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This hypothetical network has one dumb L2 switch and two hosts A and B connected to it. CAM table entries on the switch are all populated and both hosts have ARP entries for each other.

Now if a third host C gets added in the network, and is looking to go to B, it would first send ARP request with B's IP. In this case, would the switch forward the frame out of all its ports (except where it received from) since the THA (Target hardware address) is all zeros in C's ARP request packet?

My question is, even if the switch knows the hardware address of B, it still has to forward the frame out his every port just because C doesn't know the MAC address of B?

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    "would the switch forward the frame out of all its ports (except where it received from) since the THA (Target hardware address) is all zeros in C's ARP request packet?" No the target hardware address is all ones (broadcast address), not all zeros. RFC 826, An Ethernet Address Resolution Protocol explains it all. Switches send broadcasts out all interfaces except the one on which it was received. The switch has no idea that the target is Host B, only that the target is the broadcast address.
    – Ron Maupin
    May 22, 2020 at 17:13
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    @Ron The THA is part of the ARP payload; for a request it can be anything ("don't care") - which the switch doesn't know or care about, obviously. The frame's destination MAC address is all ones, of course.
    – Zac67
    May 22, 2020 at 17:32
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    @Zac67, I was referring to the target hardware address on frame since that is what the switch sees, and it is why the switch would send it to all other interfaces. The ARP packet has nothing to do with what the switch does. I guess the OP is very confused about how the switch operates.
    – Ron Maupin
    May 22, 2020 at 17:35
  • @Ron, I was talking about Target hardware address not Destination MACs. THA in ARP request are all zeros not ones. As per RFC 5227 , THA field is usually ignored and should be set to all Zeros. May 23, 2020 at 18:59
  • @ShaunakKakade, a switch has no idea that is even an ARP request, much less what the THA in the ARP request is, so that has nothing to do with flooding the frame to all other switch interfaces. Switches only look at the source and destination hardware (MAC) addresses in order to deliver a frame to any interfaces and populate it MAC address table. A switch has no idea what is in the frame payload, which is why you can use ARP, IPv4, IPX, IPv6, AppleTalk, etc. all on the same ethernet switch at the same time, and only IPv4 uses ARP.
    – Ron Maupin
    May 23, 2020 at 22:17

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