# IPv6: Path MTU Discovery failed for destination 2001:db8::1, packet size 1280

- **ID:** `networking/ipv6-mtu-path-discovery-failure`
- **Domain:** networking
- **Category:** protocol_error
- **Verification:** ai_generated
- **Fix Rate:** 85%

## Root Cause

An intermediate router is dropping ICMPv6 Packet Too Big messages (type 2) required for Path MTU Discovery, causing the sender to assume a larger MTU and leading to packet drop or fragmentation failure.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Linux 5.15+ | active | — | — |
| FreeBSD 13.2 | active | — | — |
| Cisco IOS 15.0 | active | — | — |
| Juniper Junos 21.0 | active | — | — |

## Workarounds

1. **Set the minimum MTU for the route to 1280 bytes to bypass PMTUD: ip route add 2001:db8::1/128 via fe80::1 dev eth0 mtu 1280** (85% success)
   ```
   Set the minimum MTU for the route to 1280 bytes to bypass PMTUD: ip route add 2001:db8::1/128 via fe80::1 dev eth0 mtu 1280
   ```
2. **Enable ICMPv6 filtering rules on intermediate routers to allow ICMPv6 type 2 messages (packet too big) through the firewall** (90% success)
   ```
   Enable ICMPv6 filtering rules on intermediate routers to allow ICMPv6 type 2 messages (packet too big) through the firewall
   ```

## Dead Ends

- **** — This disables IPv6 connectivity completely, which may break applications that require IPv6 and is not a targeted fix. (95% fail)
- **** — The issue is on the path, not the sender; a static MTU does not address the router dropping ICMPv6 messages. (80% fail)