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DAD Ipv6 Duplicate Address Detection


Hi everyone,

My questions are; If a node starts a DAD process and send a NS, is it creates an entry on its neighbour cache for target address? Also if it recieve a NA packet for this NS we know that target address is duplicate and we can`t assign it, but what about neighbour cache? Is this NA update neighbor cache?  

Consequently , can DAD process update neighbor cache and used for reachibilty?

All replies will be appreciated.


2 Replies 2

Phillip Remaker
Cisco Employee
Cisco Employee

See for explanation.  Section 7.2 covers this topic in detail.


Hi Gokmen,

During the TENTATIVE Step of Autoconfiguration we verify the IPv6 Addresses.

We start with Link-Local Address with DAD and if we receive a NA, two cases:

- Normal case, the interface is disabled for IPv6 (see remark below)

- SeND. We retry twice. If after 3 attempts, computed CGA are not unique we disable the Interface for IPv6 and report an Error (I tested it!)

Now about DAD itself, it is explained in rfc4862

Quite funny the RFC says that the interface is  disabled for a link-local only if it is derived from a MAC Address  otherwise IP operation MAY continue... Quite funny as I don't see how we  could continue with a duplicated Link-local address!!!!

Now when it is the turn of other Addresses, if we receive a NA, the Address is not initialized by the local Node.

AFAIK, while the interface is in TENTATIVE mode we are not supposed to build the Neighbor Cache but only verify our own addresses and other task very well defined.

The address is not initialized and may be once the interface is enabled and no longer in TENTATIVE mode we can resolve this Neighbor MAC Address but I would be surprised that it would be done while in TENTATIVE mode during DAD!!!

I stay tuned... As somebody may correct me if I am wrong!

You may be interested by page 16 and 17, click on the graphics to see the animations...

For more details about autoconfiguration you may be interested by this video (Presentation also available from slideshare, please check my wiki page)

It would be funny

Kind Regards,


15 years ccie #3013

18 years ccsi #33517 (former #95003)

IPv6 Forum Gold Certified Engineer

IPv6 Forum Gold Certified Trainer





Skype: FredericBovy

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