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When a device sends an ARP Request, it broadcasts to all Nodes, which then check against their own ARP Tables and respond with an ARP reply or drop the Packet, but what happens if the device is not directly connected?

So assume some network like this: A <-> B <-> C <-> D

If A sends an ARP request with Target-IP: D, will B cascade the request to C or just drop it?

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  • You will need to explain what at least B and C are. Routers? Switches? Hosts?
    – Teun Vink
    Oct 9, 2022 at 11:56

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what happens if the device is not directly connected?

Then that ARP request does not get a reply. (I understand "directly connected" as connected to the broadcast domain - a simple switch or a VLAN.)

An ARP request is sent in a broadcast frame. A broadcast is forwarded by a switch by sending it out of all ports but the one it was received on. Broadcasts are not forwarded by routers in general.

So assume some network like this: A <-> B <-> C <-> D [...] will B cascade the request to C or just drop it?

Assuming B and C are routers, then no. ARP requests and broadcasts generally are never forwarded by a router.

If B and C are hosts: hosts don't forward in general (if a host is configure to forward by IP it becomes a router for that purpose). If B and C are switches: those forward broadcasts normally within the broadcast domain.

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  • Ok, great, thank you. Is there a way to specify an ARP request to cascade? The ARP request fields do not specify anything, so I assume not. I just want to be sure that I understand it.
    – Alex
    Oct 9, 2022 at 13:10
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    No, broadcasts in general are never forwarded by a router. Also, learning the (or a) MAC address of a remote host is entirely useless.
    – Zac67
    Oct 9, 2022 at 13:18

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