# NDP: Duplicate address detected for fe80::1 on eth0, DAD failed

- **ID:** `networking/ndp-duplicate-address`
- **Domain:** networking
- **Category:** layer2
- **Verification:** ai_generated
- **Fix Rate:** 81%

## Root Cause

Duplicate Address Detection (DAD) for an IPv6 link-local address failed because another host on the same link is already using that address, typically due to manual configuration overlap or a rogue device.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Linux kernel 6.1.0-17-amd64 | active | — | — |
| Windows Server 2022 IPv6 stack | active | — | — |
| Cisco IOS 15.9(3)M | active | — | — |
| FreeBSD 14.0 | active | — | — |

## Workarounds

1. **Manually assign a unique link-local address on the local host: `ip addr add fe80::2/64 dev eth0` and remove the conflicting one** (90% success)
   ```
   Manually assign a unique link-local address on the local host: `ip addr add fe80::2/64 dev eth0` and remove the conflicting one
   ```
2. **Identify the conflicting host by scanning the neighbor cache: `ip -6 neigh show dev eth0`, then disconnect or reconfigure it** (85% success)
   ```
   Identify the conflicting host by scanning the neighbor cache: `ip -6 neigh show dev eth0`, then disconnect or reconfigure it
   ```

## Dead Ends

- **** — This allows duplicate addresses to persist, causing packet loss and connectivity issues for both hosts. (90% fail)
- **** — The duplicate address is still used by the other host; the local host may get a new address but the conflict is not resolved. (70% fail)
- **** — The duplicate address on the remote host remains; the local host will generate the same address again based on its MAC, leading to the same DAD failure. (80% fail)