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FULL AI ANALYSIS

Intermittent timeouts to dual‑stack websites after enabling DHCPv6 prefix delegation on OpenWrt router

Generated Oct 3, 2026, 12:35:03 AM

Analysis summary

Intermittent IPv6 reachability after enabling PD often stems from one or more of: 1) the ISP changing delegated prefixes without informing the router, leaving stale routes on the Internet or in upstream routers; 2) ICMPv6 being blocked or rate‑limited somewhere causing PMTU/Path MTU discovery failures; 3) RA/ND configuration on the router creating inconsistent address advertisements; or 4) a transient forwarding problem at the ISP or a misconfigured firewall on the router dropping traffic for newly announced prefixes. The temporary fix after reboot suggests state or cache invalidation clears the problem, consistent with stale prefix bindings or neighbor/route cache issues.

Possible causes

ISP performing prefix renumbering/PD changes without stable route propagation to peers

85%

Upstream router or carrier NAT issues where delegated prefix is not routed consistently across ISP routers

80%

Router firewall or NAT rules blocking ICMPv6 (needed for PMTU, NA/NS) causing some IPv6 flows to stall

30%

RA/ND lifetimes configured too aggressively, causing clients to use expired prefixes before route propagation completes

15%

Suggested fixes

1

Diagnostic step 1

On a client and on the router, capture packet traces during a reproduce: tcpdump -i <iface> icmp6 or tcp port 80,443 to observe whether SYNs are leaving and whether any ICMPv6 'Packet too big' or 'Destination unreachable' messages are returned.

2

Diagnostic step 2

Verify router firewall allows ICMPv6 type 1 (destination unreachable) and type 2 (packet too big), and that RA/ND are configured with reasonable lifetimes (prefer longer valid/preferred values during PD tests).

3

Diagnostic step 3

Enable prefix‑change logging on router and record timestamps of DHCPv6 PD events; if PD renumbering correlates with outages, capture DHCPv6 exchange and contact ISP with logs (escalate to ISP support with PD lease and WAN packet captures).

4

Diagnostic step 4

Temporarily bind a client with a manual global IPv6 address from the delegated prefix and attempt connections to determine whether the issue is per‑host or per‑prefix.

5

Diagnostic step 5

If PMTU suspected, test by forcing lower MTU on the client (e.g., ip -6 route change default mtu 1280) to see if timeouts stop; if so, investigate ICMPv6 filtering on path and work with ISP to permit ICMPv6 'packet too big'.

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