FAQ
Q: What should the router do in the following scenario: say a packet for a certain IP address arrives at the router and it sends an ARP request to obtain the corresponding MAC address. Before receiving the ARP reply, the router receives another packet (non-ARP) for the same IP address, should it send an ARP request again?
A: No, in this case you should not retransmit the ARP request for the second packet. More generally, your router might receive many packets for a certain IP address while there is an outstanding ARP request for that IP address. In this case, your router should not send out any new ARP requests or update the timestamp of the initial ARP request. However, your router should buffer the new data packets so that it can transmit them to the destination host once it receives the ARP reply. IMPORTANT: If your router buffers multiple packets for a destination host that has an outstanding ARP request, upon receiving the corresponding ARP reply these packets has to be forwarded to the destination host in the order they arrived to the router!
Q: When an ARP request arrives at the router for a destination IP address that is not assigned to one of the router's interfaces, does the router need to flood the ARP request, or should it just drop the request?
A: Your router should drop the packet in this case.
Q: When the router needs to make an ARP request for the next hop IP address (which is obtained after the longest prefix match lookup), should it flood the request on all ports?
A: The router does not flood the ARP request on all ports. The ARP query is merely sent to the broadcast Ethernet address on the port obtained from doing a longest prefix match lookup. The response ARP query should come back on the same port but it doesn't actually need to (and it doesn't matter for the purposes of forwarding the packet or sending out the ARP request).