Gratuitous ARP

The figure above demonstrates the use of gratuitous ARP.
When RT2 brings up its Fa0/0 interface with no shutdown interface subcommand, it would send out multiple gratuitous ARP replies to detect duplicate address before using the assigned IP address.

Below shows the operation of gratuitous ARP with the ARP debug messages on RT1 and RT2:

RT2(config-if)#no shut
RT2(config-if)#
00:04:24.035: IP ARP: sent rep src 10.10.10.1 2222.2222.2222,
                 dst 10.10.10.1 ffff.ffff.ffff FastEthernet0/0
00:04:24.039: IP ARP: sent rep src 10.10.10.1 2222.2222.2222,
                 dst 10.10.10.1 ffff.ffff.ffff FastEthernet0/0
00:04:26.011: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up
00:04:27.011: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
RT2(config-if)#
---------------------------------------------------------------------
RT1#
00:04:24.575: IP ARP: rcvd rep src 10.10.10.1 2222.2222.2222, dst 10.10.10.1 FastEthernet0/0
00:04:24.579: %IP-4-DUPADDR: Duplicate address 10.10.10.1 on FastEthernet0/0, sourced by 2222.2222.2222
00:04:24.583: IP ARP: sent rep src 10.10.10.1 1111.1111.1111,
                 dst 10.10.10.1 1111.1111.1111 FastEthernet0/0
00:04:24.587: IP ARP: rcvd rep src 10.10.10.1 2222.2222.2222, dst 10.10.10.1 FastEthernet0/0
00:04:24.587: IP ARP: Gratuitous ARP throttled.
00:04:24.591: IP ARP: 10.10.10.1 added to arp_defense_Q
00:04:25.139: IP ARP: 10.10.10.1 removed from arp_defense_Q
00:04:25.139: IP ARP: sent rep src 10.10.10.1 1111.1111.1111,
                 dst 10.10.10.1 1111.1111.1111 FastEthernet0/0
RT1#

Download the packets captured on RT1 FastEthernet0/0 here.