Managing Duplicate Address Detection
The Duplicate Address Detection (DAD) feature checks networks attached to a switch to see if IP addresses configured on the switch are already in use on an attached network. The following sections provide additional information on the DAD feature:
When you configure an active interface with an IPv6 address, the interface must send out an advertisement containing its address.
All other interfaces on the subnet have the opportunity to respond to the newly configured interface, and inform it that the address is a duplicate. Only after this process occurs, can the interface use the newly configured address. If the interface receives a message that the newly configured address is a duplicate, it cannot use the address.
Until the Duplicate Address Detection (DAD) process completes, the new address is considered tentative, and will be shown as such in any display output. If the address is a duplicate, it will also be labeled as such, and must be reconfigured. On an active interface, the DAD process should occur so quickly that you would not see the address labeled as tentative. However, if you are configuring an interface before enabling it, and you display the configuration, you will see that the address is currently tentative. As soon as you enable the interface, the address should be ready to use, or labeled as duplicate and must be reconfigured.
See RFC 2462, IPv6 Stateless Address Autoconfiguration, for more details.