Duplicate Address Detection
All nodes perform duplicate address detection before assigning a unicast address to an interface. It is not performed for anycast addresses. This is performed regardless of whether the address is assigned via stateless, stateful, or manual configuration. It is performed before assigning an address to an interface and on an initializing interface. The address to be assigned to the interface is called "tentative" while the duplicate address detection process is taking place.
Before sending a solicitation, the interface joins the all-nodes multicast group to ensure that the node receives Neighbor Advertisements from any node already using the address and joins the solicited-node multicast group for the tentative address to ensure that if another node is attempting to begin using the address, both nodes will learn of each other's presence.
The node sends a Neighbor Solicitation message, with the tentative IP address as the target. The source address is the unspecified address, and the destination is the tentative address's solicited-node multicast address. By default, one solicitation is sent.
Any neighbor that is already assigned the address receives the solicitation and sends a Neighbor Advertisement in reply. The target specified in the advertisement is the tentative address. The destination address is the solicited-node address of the tentative address. If a node receives this Neighbor Advertisement, and the target address is the interface's tentative address, the address is a duplicate and must not be assigned to the interface. Some IPv4 hosts perform a duplicate address detection process before assigning an IP address to an interface. Not all do, however, allowing an interface with a duplicate address to potentially disrupt existing traffic flows.
Continue reading here: MTU Path Discovery
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