When administratively enabled on a VR, Duplicate Address Detection (DAD) runs when an IPv6 address is added to a VLAN, the VLAN transitions to an operational state, or when the first valid link-local address is added to the VLAN.
DAD works by sending a neighbor solicitation for each IPv6 address configured on a VLAN. If another device replies to the neighbor solicitations by sending neighbor advertisements for any of these IPv6 addresses, the referenced IPv6 addresses are marked duplicate and are not used. If no responses are received, the IPv6 addresses become active and available for subsequent use. Once an IPv6 address becomes active, DAD does not run again for that address unless that address is deleted and re-added, the VLAN goes down and comes back up, or the first valid link-local address is added to the VLAN.
IPv6 functionality requires at least one valid link-local address. If the last valid link-local address is marked duplicate, all non-duplicate IPv6 addresses become tentative.