How many broadcasts are needed in ARP protocol for one new device send to another new device to get necessary ARP table entries for lookup in one switch?

  • Switches do not use ARP tables, the hosts do. Switches use MAC address tables. – Ron Maupin Dec 18 '20 at 13:45
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could post and accept your own answer. – Ron Maupin Dec 31 '20 at 5:11

A simple L2 switch doesn't have an ARP table. It has a MAC address table. Once a switch accepts a frame on one of its ports it registers the source MAC address to the port the frame arrived from, so that future frames that are destined to that MAC address can be delivered to that specific port (and not flooded to all ports). ARP is just one type of packet (could be any other type) that enables the switch to learn the information it needs. Specifically for ARP, it's enough to have an ARP request to learn the MAC address of device 1, and an ARP reply to learn the MAC address of device 2 (assuming such device exists on the same broadcast domain). L3 switches also have their own ARP tables for management purposes (enables you to connect to the switch via SSH) and for routing.

  • In total for two new devices, does that mean I will only make switches broadcast two messages to get all information in the ARP table (for the two new devices), In one switch, I mean two devices in the same Ethernet switch. – 7337dtd Dec 18 '20 at 6:49
  • The devices should have their local ARP tables. After they have MAC address to IP address mapping they can start communicating. In this context the switch is only a means for delivering the packets on the local segment. The switch doesn't care about MAC to IP address mappings. – manish ma Dec 18 '20 at 7:07

How many broadcasts are needed in ARP protocol for one new device send to another new device to get necessary ARP table entries

One. The node trying to send an IP packet broadcasts an ARP request and hopefully receives a reply, learning the required MAC address for link-layer transport.

for lookup in one switch?

The ARP process above also ensures MAC table population in all involved switches.

Switches themselves don't use ARP. They just require the destination MAC that the sender learned through ARP for IPv4.

Switches only use ARP for layer-3 functions (when supported) like management over IPv4, or layer-3 switching/routing.

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