hi

[sajeer9745]

what is ARP table and how a switch uses ARP table?

[David Johan Castro Fernandez]

Hello,

ARP --> Address Resolution Protocol

ARP uses layer 3 (IP addresses) to resolve layer 2 (MAC addresses) , so basically The ARP table will tell you the IP to MAC address mapping.

A host wishing to obtain a physical address broadcastsan ARP request onto the TCP/IP network. The host on the network that has the IP address in the request then replies with its physical hardware address.

There is also Reverse ARP (RARP)which can be used by a host to discover its IP address. In this case, the host broadcasts its physical address and a RARP server replies with the host's IP address. 

So basically that is the used for it, now it also works for troubleshooting purposes, you can see what is the MAC address of the IP address assigned on a host that is a neighbor device.

Please Don't forget to rate and mark as correct the helpful Post!

David Castro,

[Joseph W. Doherty]

An ARP table is a time based cache of IP addresses and their corresponding MAC.

Its purpose is to avoid the need to broadcast an ARP to resolve the MAC for every packet's destination MAC.

A switch would use the ARP cache/table like any other host. I.e. if it wants to send a packet to an IP, it 1st checks if that IP is in the ARP cache/table, and if so, it uses the MAC for the frame's destination.

BTW, since you're asking about a "switch", you might be confusing the ARP cache/table with a switch's MAC table.

Also BTW, "dumb" switches wouldn't have an ARP cache/table.

[Richard Burts]

I would like to emphasize a point that Joseph makes. This question assumes that we are talking about switches that operate only as layer 2 switches. A layer 3 switch with routing enabled maintains and uses an arp table in the same way that a router does. A switch operating as a layer 2 switch forwards traffic passing through the switch using the layer 2 mac address table and does not need an arp table for transit traffic. The layer 2 switch does maintain and use an arp table for traffic that is originated by the switch itself. In that sense the switch is acting as a host and uses arp in the same way that a host does.

HTH

Rick