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i know that this might be a little academic, but i was wondering. let's assume we have an empty ARP table, and empty MAC Table. let's assume PC A want to send a packet to PC B.

is the broadcast frame sent as a broadcast due to the ARP destenation or does it get flooded to all connected ports due to the empty MAC Address table. who'se "fault" is it ?

my guess is that it is because of the empty MAC TAble, the switch sent floods the frame, regardless to the ARP Dest. address

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  • An ARP request is always broadcast. If you knew the MAC you wouldn't need to ask.
    – Zac67
    Oct 5, 2022 at 17:45
  • i know that. i'm asking someting else... Oct 5, 2022 at 17:47
  • The switch does not decide if it is a broadcast. The host sending the ARP request sends it to the broadcast address.
    – Ron Maupin
    Oct 5, 2022 at 17:57
  • if the MAC Table on the switch is empty, then the switch will send the frame as broadcast, regardless if the frame dest is for a specific host or broadcast. Oct 5, 2022 at 17:59
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    No, that is not true. An unknown unicast frame is flooded, not broadcast. Broadcast implies use of the broadcast address, but unknown unicast frames still use unicast destinations.
    – Ron Maupin
    Oct 5, 2022 at 18:01

1 Answer 1

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is the broadcast frame sent as a broadcast due to the ARP destenation

An ARP request's destination address is always the broadcast address.

or does it get flooded to all connected ports due to the empty MAC Address table.

The MAC address table in a switch is irrelevant for a broadcast frame. Broadcasts are always forwarded to all ports but the one they were received on.

You seem to be confusing broadcast forwarding with flooding. On a switch, the latter has the same result as the former, but flooding is due to the switch's inability to select the correct egress port when it hasn't learned a frame's unicast destination address (yet). Same result, different mechanisms. Think of flooding as a contingency measure (my mimicking a repeater hub) and broadcasting as a normal mechanism.

Flooded frames are generally ignored by connected host NICs, except for the one being addressed (or the ones being in promiscuous mode). This contrasts with broadcast frames which are received by all connected nodes.

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  • thanks. so, if i will sum up: the switch floods the frame to all ports. the broadcast address in the arp request - allows all hosts to recieve the frame and examine it. Oct 5, 2022 at 18:22
  • Still, an ARP request('s frame) is broadcast by a switch, not flooded.
    – Zac67
    Oct 5, 2022 at 18:26
  • frame destination is broadcast address means deliver frame to all systems on the same Ethernet segment. on the switch this is implemented by flooding the frame on each port, because how else would you deliver a frame to each system. broadcast addresses never go into MAC tables, because it is broadcast. there are no faults here.
    – Effie
    Oct 6, 2022 at 9:00
  • generally, frames that have destinations set to broadcast addresses are handled differently from frames that are destined to unicast addresses.
    – Effie
    Oct 6, 2022 at 9:03

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