Duplicate address detection

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

Duplicate address detection is the process of determining whether an IPv6 address is already in use on a link before assigning it to an interface.

Duplicate Address Detection (DAD) checks whether an IPv6 address is already in use before an interface communicates with it. The address remains tentative until the check finishes. DAD runs when an address is configured and whenever an interface initializes. section 21.1.4

DAD follows this sequence:

• Send a Neighbor Solicitation (NS) message to the address's solicited-node multicast address. section 21.1.4
• Wait for a response. No response allows the host to mark the address unique and use it. section 21.1.4
• Treat a Neighbor Advertisement (NA) response as evidence of a duplicate; the address cannot be used. section 21.1.4

Figure 21.4 R2 uses DAD to verify that its Ipv6 address is unique. (1) Configure R2’s address. (2) R2 sends an NS message to its own solicited-node multicast address. (3) R2 waits for a response. (4) R2 doesn’t receive a response, so the address is unique.

Follow the router scenario from address configuration through the NS probe, waiting period, and uniqueness decision. section 21.1.4

Cisco duplicate-address warning

After configuring an IPv6 address and enabling the interface, Cisco IOS can display a duplicate-address warning. The address appears as [DUP] in show ipv6 interface. Remove the conflicting address with no and configure a unique address. section 21.1.4

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Explains the NS/NA exchange, tentative state, duplicate result, and Cisco IOS symptom without detailing message fields or timers.

Examples

Uses a Cisco router configuration, duplicate warning, show ipv6 interface output, and the [DUP] status.

What it assumes

Assumes basic IPv6 interface configuration while defining NDP messages, tentative addresses, and solicited-node multicast behavior.

What you can do after

Diagnose duplicate IPv6 addresses from IOS warnings and [DUP] output, then remove the conflict and configure a unique address.

How it connects

Connects IPv6 addressing with Neighbor Discovery Protocol, Neighbor Solicitation, Neighbor Advertisement, and solicited-node multicast addresses.

liveBookmanning.com

Read Acing the CCNA Exam: Volume 1 Fundamentals and Protocols on liveBook Get the book at manning.com

Related concepts

• neighbor solicitation messageuses · A Neighbor Solicitation is an ICMPv6 Neighbor Discovery message used to request link-layer information or verify the reachability of a neighboring node.
• Neighbor Discovery Protocolpart of · Neighbor Discovery Protocol is an IPv6 protocol that resolves neighboring IPv6 addresses to link-layer addresses and supports functions such as router discovery and duplicate address detection.
• neighbor advertisementuses · A Neighbor Advertisement is an ICMPv6 message that communicates a neighbor's link-layer address or indicates that an IPv6 address is already in use.
• IPv4contrasts with · IPv4 is the fourth version of the Internet Protocol, using 32-bit addresses to identify network interfaces and packet sources or destinations.
• detectionkind of · Detection is the process of identifying whether a specified condition, abnormality, failure, or undesirable behavior is present or suspected.
• IPv6uses · IPv6 is the sixth version of the Internet Protocol, using 128-bit network addresses for packet-based communication.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.