IPv6CCNA 200-301

IPv6 Duplicate Address Detection (DAD) Failure

Presenting Symptom

A host fails to obtain an IPv6 address via SLAAC or DHCPv6, and the network engineer observes 'Duplicate Address Detection' failures in the logs.

Network Context

A small branch office with a Cisco router (IOS XE 16.9) acting as the default gateway and a few Windows 10 hosts. The router is configured to advertise IPv6 prefixes via SLAAC. One host consistently fails to configure its IPv6 address, while others work fine.

Diagnostic Steps

Check IPv6 interface status and DAD attempts

show ipv6 interface GigabitEthernet0/0

GigabitEthernet0/0 is up, line protocol is up
  IPv6 is enabled, link-local address is FE80::1
  No global unicast address is configured
  Duplicate address detection: 1 duplicate address detected
  ...

If 'Duplicate address detection' shows a non-zero count, DAD has detected a duplicate. The interface may be in 'tentative' state or have no global address.

Identify the duplicate address

show ipv6 neighbors

IPv6 Address                              Age Link-layer Addr State Interface
2001:DB8:1::100                              0   aaaa.bbbb.cccc  REACH    Gi0/0
2001:DB8:1::100                              0   dddd.eeee.ffff  STALE    Gi0/0

Two entries with the same IPv6 address but different MAC addresses indicate a duplicate. The duplicate address is 2001:DB8:1::100.

Check the router's own IPv6 address configuration

show running-config | section interface GigabitEthernet0/0

interface GigabitEthernet0/0
 ipv6 address 2001:DB8:1::1/64
 ipv6 enable
 no ipv6 nd suppress-ra

The router has a static IPv6 address. Ensure it is not conflicting with the host's address. The host's tentative address is 2001:DB8:1::100, which is different from the router's 2001:DB8:1::1.

Check for duplicate MAC addresses on the LAN

show mac address-table interface GigabitEthernet0/0

Mac Address Table
-------------------------------------------
Vlan    Mac Address       Type        Ports
----    -----------       --------    -----
   1    aaaa.bbbb.cccc    DYNAMIC     Gi0/0
   1    dddd.eeee.ffff    DYNAMIC     Gi0/0

Multiple MAC addresses on the same port is normal for a switch, but if the same MAC appears on different ports, it could indicate a loop or duplicate MAC. However, here two different MACs are seen, which is fine.

Root Cause

Two hosts on the same VLAN have been manually configured with the same static IPv6 address (2001:DB8:1::100). When one host attempts to use that address, DAD detects the duplicate and prevents the address from being assigned, causing the host to fail to obtain a valid IPv6 address.

Resolution

1. Identify the conflicting host by checking its MAC address from the neighbor table. 2. On the conflicting host, change the IPv6 address to a unique one, or enable DHCPv6 for automatic assignment. 3. Alternatively, on the router, you can clear the duplicate neighbor entry and then configure the host with a different address. Commands on the router: clear ipv6 neighbors (Then reconfigure the host with a unique address) On the host (Windows): netsh interface ipv6 set address "Local Area Connection" 2001:DB8:1::101

Verification

After resolving the duplicate, run: show ipv6 interface GigabitEthernet0/0 Expected: 'Duplicate address detection: 0 duplicate addresses detected' show ipv6 neighbors Expected: Only one entry for 2001:DB8:1::100 (or the new address) with the correct MAC. ping 2001:DB8:1::100 Expected: Successful replies.

Prevention

1. Use DHCPv6 for address assignment to avoid manual configuration errors. 2. Implement IPv6 address management (IPAM) to track assigned addresses. 3. Enable DAD on all interfaces (default) and monitor logs for duplicate address warnings.

CCNA Exam Relevance

On the CCNA 200-301 exam, DAD is tested in the context of IPv6 addressing and troubleshooting. Questions may present a scenario where a host cannot get an IPv6 address, and you must identify DAD failure as the cause. Expect multiple-choice or simulation questions where you interpret 'show ipv6 interface' output to see duplicate address count.

Exam Tips

Remember that DAD is performed using Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages.

The 'show ipv6 interface' command shows the number of duplicate addresses detected; a non-zero value indicates a problem.

A duplicate address causes the interface to remain in 'tentative' state and the address is not assigned.

Commands Used in This Scenario

show ipv6 neighbors

Displays the IPv6 neighbor discovery cache, showing the mapping of IPv6 addresses to MAC addresses on directly connected links, used to verify neighbor reachability and troubleshoot IPv6 connectivity.

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