The ARP table on a Cisco device is a list of learned IP address and what MAC addresses they resolve to, this is required as generally switches work at layer 2 with MAC addresses not IP Address’s.

Problem:

If you replace a device with another device that is still using the same IP address as the original device but has a different MAC address you find that you are unable to connect to it due to the ARP record on your Cisco device is still pointing to the original devices MAC address.

Example:

I have replaced a failed printer which has the IP 10.24.100.54 and the MAC address 00:80:77:b6:b:207 with a hot spare printer that has been setup to have the same IP 10.24.100.54 however its MAC address is 00:80:77:82:d9:12, the printer is all connected and configured fine however I am unable to ping or print.

Resolution:

1) Get yourself into the enable mode on the Cisco switch the device is connected to.

2) Enter the below command to see the entry in the ARP table for the IP 10.24.100.54.

sh ip arp | include 10.24.100.54

Internet 10.24.100.54 23 0080.77b6.b207 ARPA Vlan124

Notice that the MAC address stored is still the original device

Enter the below command to clear this single dynamic entry

clear ip arp 10.24.100.54

If you attempt to ping you device again you should find everything is fine and if you run the show command again you should see the entry in the ARP table now has the new MAC address

Internet 10.24.100.54 1 0080.7782.d912 ARPA Vlan124

If you are making a lot of changes you could also run the command clear arp-cache to clear the entire table.

Author: Phil Eddies

I am an IT Operations Manager, managing all aspects of the IT infrastructure and service for a mid sized UK based company. I have been working full time in IT since 2001 in 1st to 3rd line and System Administration roles. MCSA, MCSE, CCNA, Citrix CCA