DAD operation

On a given VLAN interface, when a new unicast address is configured, the switch runs DAD for this address by sending a neighbor solicitation to the All-Nodes multicast address (ff02::1). This operation discovers other devices on the VLAN and verifies whether the proposed unicast address assignment is unique on the VLAN. (During this time, the address being checked for uniqueness is held in a tentative state and cannot be used to receive traffic other than neighbor solicitations and neighbor advertisements.) A device that receives the neighbor solicitation responds with a neighbor advertisement that includes its link-local address. If the newly configured address is from a static or DHCPv6 source and is found to be a duplicate, it is labeled as duplicate in the "Address Status" field of the show ipv6 command and is not used. If an autoconfigured address is found to be a duplicate, it is dropped and a similar message appears in the Event Log:

W <date> <time> 00019 ip: <ip address> <IPv6-address> removed from vlan id <vid>

DAD does not perform periodic checks of existing addresses. However, when a VLAN comes up with IPv6 unicast addresses configured (as can occur during a reboot), the switch runs DAD for each address on the interface by sending neighbor solicitations to the All-Nodes multicast address, as described previously.

If an address is configured while DAD is disabled, the address is assumed to be unique and is assigned to the interface. If you want to verify the uniqueness of an address configured while DAD was disabled, re-enable DAD and then either delete and reconfigure the address, or reboot the switch.