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Users can reach an intranet site by IPv4 but not by IPv6 — the site is dual-stacked, DNS returns both A and AAAA records. IPv4 works, IPv6 silently fails after a five-second delay. This is the classic Happy Eyeballs problem, and diagnosing it takes discipline.
IPv6 failures are frequently masked by IPv4 fallback. Modern OSes try IPv6 first and fall back to IPv4 on failure — which means users see slow connections instead of errors. You have to test IPv6 explicitly to find the fault.
`No route` from the router means the router has no route to `2001:db8:acad:99::/64`. The client is fine — its own addressing and ND worked (it got a response from the router). The failure is upstream routing.
| Symptom | Likely cause | Check |
|---|---|---|
| Host has only link-local | No RA received, or router not advertising | `show ipv6 interface` on router |
| Global address but no default route | RA without default gateway info, or RA being filtered | RA flags, ICMPv6 firewall rules |
| Ping by IP works, ping by name fails | DNS missing AAAA record or resolver issue | `dig AAAA hostname` |
| Works to some destinations, not others | Routing problem — missing static or dynamic route | `traceroute6`, `show ipv6 route` |
| Slow connections but no errors | IPv6 trying and failing before IPv4 fallback | `ping -6`, disable IPv6 temporarily to confirm |
| Link-local ping works, global doesn't | Path MTU or filtering issue | `ping -6 -s 1400`, check ICMPv6 firewall |
IPv6 requires a minimum link MTU of 1280 bytes, and routers do not fragment on behalf of hosts. If a link has a smaller MTU, the router sends an ICMPv6 Packet Too Big message back to the source. If that ICMPv6 is blocked, the source never learns to send smaller packets, and large transfers fail while small pings succeed.
`Message too long, mtu=1280` tells you the path MTU is 1280 — the IPv6 minimum. If this fails and a smaller size works, you're hitting a path MTU issue. Fix by ensuring ICMPv6 Packet Too Big messages aren't filtered, or by adjusting application-level MSS clamping.
The single biggest IPv6 troubleshooting mistake is not testing IPv6 explicitly. If you only `ping google.com`, you're testing IPv4. Use `ping -6` (or `ping6`) to force IPv6 and see the real behavior.
Section 4 wraps. IPv6 addressing, address types, NDP, SLAAC, routing, and troubleshooting — the full picture. Section 5 shifts to Cisco IOS and device management — where you'll actually configure all of this on real gear.
Finish the lesson once you have worked through the material. This awards ★ 30 XP.