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Can you confirm that if host 1 wants to send a broadcast packet, he will send it to the router first, and the router will send it to all machines to the LAN ?

I need to clarify this point in order to understand dynamic arp inspection.

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  • A router is just another host on the LAN. – Ron Maupin♦ Jun 17 '16 at 3:21
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Can you confirm that if host 1 wants to send a broadcast packet, he will send it to the router first, and the router will send it to all machines to the LAN ?

This is incorrect. Broadcasts are sent by a host to all other hosts on the same subnet (PCs, phones, routers, switches, etc) at the same time. The destination MAC for a broadcast is FF:FF:FF:FF:FF:FF. If the packet was to be sent to a router or switch the destination MAC would be that of the router or switch and it would be a unicast packet as opposed to a broadcast packet.

Now having said the above, in order for a broadcast packet to reach the other devices it will need to go through at least one switch. When the switch sees the destination MAC of FF:FF:FF:FF:FF:FF it usually sends the packet out all of its interfaces which are in the same VLAN, except for the interface the packet was received on. However, when Dynamic ARP inspection is enabled the switch is able to drop the packet instead of sending it out all interfaces if it deems it to be a malicious ARP packet.

See this page for more information.

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  • 1
    Thanks. When you say "in order for a broadcast packet to reach the other devices", you means other devices from another subnet right? – Duke Nukem Jun 16 '16 at 23:33
  • @DukeNukem No. Broadcasts are contained within a broadcast domain (layer 2 domain). This means that a broadcast will only reach other hosts in the same VLAN as the source of the broadcast. Further to this, ARPs are only used to find the MAC address of hosts in the same subnet as the source. If you need to reach a hots in another subnet you'd send an ARP request for your default gateway and then send it your traffic. It will then forward your traffic towards the destination host. – OzNetNerd Jun 17 '16 at 1:31

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