What is ARP and Mac-addres table?

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Jun 8th, 2009

what is the difference between sh arp and Sh mac-address table. One entery that is present in ARP will be it present in MAC-address table also. And one entery that is present in Mac will it present in ARP also?

I have this problem too.
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Jon Marshall Mon, 06/08/2009 - 03:06


The mac-address table shows the mac-address to port/interface mappings ie. which mac-addresses are on which ports on a switch.

The arp table shows the mapping between the IP address and the mac-address ie. a device with this IP address has this mac-address.


Joseph W. Doherty Mon, 06/08/2009 - 03:55

The ARP table would be a cache of IP to MAC translations that the host has sent IP traffic to (recently - within cache timeout). For something like a switch, there should be ARP table entries for the switch interacting with telnet/SSH from its management address, or perhaps sending SNMP replies or traps to a NMS, or perhaps logging to a syslog server, etc.

The MAC table is what the switch uses to keep track of what MACs are known per port. This so a L2 frame that enters the switch, with a unicast MAC destination address, doesn't have to be flooded to all ports.

Richard Burts Mon, 06/08/2009 - 06:47


One way to understand the difference between the mac-address-table and the arp table is that the mac-address-table is used for layer 2 forwarding while the arp table is used for layer 3 forwarding. This difference in purpose helps explain the different way that these tables are built. The mac-address-table is built by examining the source mac address of every layer 2 frame entering the switch and making sure that this mac address is in the mac-address-table and adding it to the table if the mac address is not already in the table. The arp table is built when a layer 3 device wants to talk to some other layer 3 device (in the same subnet) and sends an arp request. If the target device responds then an entry is created in the arp table.

Another difference is that the default timeout for entries in the mac-address-table is much shorter than the default timeout for entries in the arp table. This can lead to situations where there is an entry in the arp table but no entry for that mac address in the mac-address-table. This can lead to unicast flooding as Joseph mentions.




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