What is Gratuitous ARP ?

[nitinm001]

Can any one please tell me what is Gratuitous ARP.. ?

[Mike Kane]

It's a crafted ARP packet where the host sends out an  ARP for it's own IP address.   This will check for Duplicate IPs on the network. 

[Richard Burts]

I do not agree that gratuitous arp is when you arp for your own address. I do agree that arp for your own address is a valid thing to do if you want to check for possible duplicate address.  But that is not gratuitous arp.

We normally think of arp in terms of some device sends an arp request and the device will send an arp response. Gratuitous arp is when a device will send an arp reply that is not a response to a request. Depending on the particular IP stack, some devices will send gratiutous arp when they boot up, which announces their presence to the rest of the network. Many devices will send an arp response (gratuitous arp) if you change the IP address of the interface.

HTH

Rick

[nitinm001]

Thanks Richard for your answers...helps me a lot..

[cp.moncy123]

In a fail over context for instance HSRP, VRRP etc.. the GARP update the ARP table on all the switches that falls on the same broadcast domain. But no changes to end user or the domain outside to the current broadcast where the fail over occurred.

[ben.carbery]

This is incorrect.

Gratuitous ARPs can be either requests or replies depending on implementation, but from sampling a large network, they are usually requests.

Specifically they are a request or a reply where the sender IP and target IP fields are the same and are the address of the transmitter.

Receiving systems will then associate the sender IP with the sender MAC.

[rsijori@]

I agree 100%, I am seeing them in my wireshark as we speak :)

[nitinm001]

Thanks for your reply

[flexkikr11]

Gratuitous ARP is a sort of "advance notification", it updates the ARP cache of other systems before they ask for it (no ARP request) or to update outdated information.

When talking about gratuitous ARP, the packets are actually special ARP request packets, not ARP reply packets as one would perhaps expect. Some reasons for this are explained in RFC 5227.

The gratuitous ARP packet has the following characteristics:

• Both source and destination IP in the packet are the IP of the host issuing the gratuitous ARP
• The destination MAC address is the broadcast MAC address (ff:ff:ff:ff:ff:ff)
  • This means the packet will be flooded to all ports on a switch
• No reply is expected

Gratuitous ARP is used for some reasons:

• Update ARP tables after a MAC address for an IP changes (failover, new NIC, etc.)
• Update MAC address tables on L2 devices (switches) that a MAC address is now on a different port
• Send gratuitous ARP when interface goes up to notify other hosts about new MAC/IP bindings in advance so that they don't have to use ARP requests to find out
• When a reply to a gratuitous ARP request is received you know that you have an IP address conflict in your network

As for the second part of your question, HSRP, VRRP etc. use gratuitous ARP to update the MAC address tables on L2 devices (switches). Also there is the option to use the burned-in MAC address for HSRP instead of the "virtual"one. In that case the gratuitous ARP would also update the ARP tables on L3 devices/hosts.

HTH,

Paul

[Theodore.Foster]

Great explanation.  Really easy to understand. You should write some cert prep books.

[Richard Burts]

I am glad that you found this discussion helpful. For a while I was a certified Cisco instructor and taught a variety of Cisco classes. I enjoy sharing what I have learned with others in the networking field and these forums are a way for me to continue to do this now that I am not in the classroom very much.

HTH

Rick

[haiganwa]

Your answer has helped so many people, thank you for sharing

[michal.dorosz]

Thanks, Paul.