![]() ![]() 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. Other machines maintain an ARP table that contains the MAC associated with an IP. Clustering solutions utilize this when they move an IP from one NIC to another, or from one machine to another. They assist in the updating of other machines' ARP table. When a machine receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict. Gratuitous ARPs are useful for four reasons: A gratuitous ARP reply is a reply to which no request has been made. A gratuitous ARP request is an AddressResolutionProtocol request packet where the source and destination IP are both set to the IP of the machine issuing the packet and the destination MAC is the broadcast address ff:ff:ff:ff:ff:ff. Gratuitous in this case means a request/reply that is not normally needed according to the ARP specification ( RFC 826) but could be used in some cases. ![]() Gratuitous ARP ( RFC 5227) could mean both gratuitous ARP request or gratuitous ARP reply. ![]()
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