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55246 Views 6 Replies Latest reply: Aug 5, 2011 10:22 AM by Elvin Arias RSS

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Gratuitous ARP

Aug 2, 2010 6:42 AM

Hi all


Gratuitous ARP is used when hosts need to update other local host ARP tables, check for duplicate IP address. When I google, I see sites telling that both Gratuitous ARP request and replies are being used.


As per the given below link Gratuitous ARP request Opcode: request (0x0001) is sent to check for duplicate IP address.



I tried the following two cases:


  • shut down/ no shut an interface
  • Configured an IP address for an interface


For both the case, I observed using Wireshark that Gratuitous ARP reply - Opcode: reply (0x0002) is sent by the router.



Ethernet II, Src: 02:02:02:02:02:02, Dst: ff:ff:ff:ff:ff:ff
    Destination: ff:ff:ff:ff:ff:ff (Broadcast)
    Source: 02:02:02:02:02:02 (02:02:02:02:02:02)
    Type: ARP (0x0806)
    Trailer: 000000000000000000000000000000000000
Address Resolution Protocol (request/gratuitous ARP)
    Hardware type: Ethernet (0x0001)
    Protocol type: IP (0x0800)
    Hardware size: 6
    Protocol size: 4
    Opcode: request (0x0002)
    Sender MAC address: 02:02:02:02:02:02 (02:02:02:02:02:02)

    Sender IP address: (
    Target MAC address: ff:ff:ff:ff:ff:ff (Broadcast)

    Target IP address: (




Are Gratuitous ARP request Opcode: request (0x0001) being used or not? If yes, can someone please let me know when they are used?




With regards



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