5 Replies Latest reply: Oct 12, 2011 3:33 PM by chemilio RSS

    ARP and ARP Cache

    Vijay Swaminathan

      Dear All,

       

      I would like to understand under what situations we would be using ARP Cache.

       

      From my initial reading, both ARP and ARP Cache seems to store the IP address to MAC mapping but I could not get exactly why and when do we use ARP Cache and ARP table?

       

      can somebody shed some tight on this please.

       

      Thanks.

         
        • 1. Re: ARP and ARP Cache
          Kumar Mantri

          Hi,

           

               Both are same, only difference is static and dynamic ,most of the time, we use dynamic ARP entries ,means is that the ARP entry (the Ethernet MAC to IP address link) is kept on a device for some period of time, as long as it is being used. The opposite of a dynamic ARP entry is static ARP entry. With a static ARP entry, you are manually entering the link between the Ethernet MAC address and the IP address. Because of management headaches and the lack of significant negatives to using dynamic ARP entries, dynamic ARP entries are used most of the time.

           

          suppose if we have pc connected to a device, as long as the device is connected we can see the arp entry in the pc , if we remove the cable between pc and device the arp entry will be removed ..called dynamic

          • 2. Re: ARP and ARP Cache
            cadetalain

            Hi,

             

            ARP table and ARP cache is the same concept but different names.

            The device first looks into its arp cache to see if it has a mapping to be able to create the L2 header with DST MAC.If it doesn't have then it will do an ARP request and when it receives reply put it for a certain amount of time in its ARP cache so subsequent communication to the device having this MAC will be possible without each time sending an ARP request.

             

            Alain.

            • 3. Re: ARP and ARP Cache
              Vijay Swaminathan

              Hi Alain,

               

              thanks for your response.. so "show arp" output what are we referring to? it is the arp cache or arp table?

               

              if we look at only arp cache for creating the L2 header with DST mac, when will we look at arp table?

               

              Do the arp table have timeout concept as well ? if so how long??

              • 4. Re: ARP and ARP Cache
                Kumar Mantri

                Hi Vijay,

                     when u give "show arp" , u are reffering to both, means both are same,only one table for ARP not two.

                • 5. Re: ARP and ARP Cache
                  chemilio

                  Hi Vijay,

                   

                  ARP is _the_ protocol! The protocol reserves a temporary storage place in RAM memory of the device called the arp-cache, this is basically where the arp-table is being stored/maintained. So when you do an show arp you are retrieving the arp-table from the arp-cache/storage.

                   

                  Just some usefull information on how ARP works!

                   

                  http://www.watchguard.com/infocenter/editorial/135324.asp

                   

                  ARP, a very simple protocol, consists of merely four basic message types:

                   

                  1. An ARP Request. Computer A asks the network, "Who has this IP address?"
                  2. An ARP Reply. Computer B tells Computer A, "I have that IP. My MAC address is [whatever it is]."
                  3. A Reverse ARP Request (RARP). Same concept as ARP Request, but Computer A asks, "Who has this MAC address?"
                  4. A RARP Reply. Computer B tells Computer A, "I have that MAC. My IP address is [whatever it is]"

                   

                  A step by step how ARP works:

                  http://sites.google.com/site/amitsciscozone/home/important-tips/arp/how-arp-wo

                   

                  Chemillio.