How does Gratuitous ARP packets work?

marinam · Jul 17, 2009

Hello,

I have the following question.

A windows machine, when starting it will broadcast 3 Gratuitous ARP packets.

These packets are broadcasted to all networks. When another station in the network receives this broadcast, it will automatically update its ARP cache?

I know that with Regular ARP this is not the case, but what about Gratuitous ARP?

I am a bit confused because i read the following in the wireskark FAQ:

Gratuitous ARPs are useful for four reasons:

They assist in the updating of other machines' ARP tables. Clustering solutions utilize this when they move an IP from one NIC to another, or from one machine to another. Other machines maintain an ARP table that contains the MAC associated with an IP. When the cluster needs to move the IP to a different NIC, be it on the same machine or a different one, it reconfigures the NICs appropriately then broadcasts a gratuitous ARP reply to inform the neighboring machines about the change in MAC for the IP. Machines receiving the ARP packet then update their ARP tables with the new MAC.

The above, is it just in the case that we have Clusters? Because I run Wireshark and I can not see any updates in any ARP cache in my systems when a Gratuitous ARP is firsly broadcasted.

Thank you, any help will be appreciated

How does Gratuitous ARP packets work?

marinam

Newbie level 3

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How does Gratuitous ARP packets work?

marinam

Newbie level 3

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