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CCNA · 200-301Intermediate

IPv6 Neighbor Discovery: Default Gateway Failure

A host has a SLAAC address but no IPv6 default route. Diagnose it and fix it yourself, live, right below.

Interactive simIPv6Neighbor DiscoverySLAAC

Symptom

Hosts have a global IPv6 address but cannot leave the LAN. Diagnose router advertisements and the default-router lifetime.

Your mission

Prove the cause, then fix it

Format

Guided CLI investigation

Start interactive labSee the investigation plan ↓Need a hint? Reveal the root cause ↓
IPv6 Neighbor Discovery: Default Gateway Failure

Investigate before you configure.

Use the CLI evidence to isolate the fault, make the smallest safe correction, then verify the network state changed.

Observe the symptom and link state.

Diagnose by comparing the protocol evidence.

Verify the expected device state and confirm the original symptom is resolved.

Console access: R1

This workspace stacks for portrait phones. Rotate to landscape for the full split-screen console.

Need a hint?

Reveal the root cause when you're ready.

Try the investigation first, then use this as your escape hatch, not a dead end.

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Lab debrief

Turn the session into a repeatable troubleshooting pattern.

Use this reference after your attempt: first explain the symptom, then verify the evidence, then confirm the repair.

The problem

A host has a SLAAC address in the correct /64 but no IPv6 default route because the router's advertisements are suppressed on the user-facing interface.

What you'll practice

  • Inspect IPv6 host routes
  • Explain router-advertisement default-router information
  • Verify ND state
  • Restore off-link IPv6 connectivity

The topology

A router serves one user VLAN and provides the only path to an upstream IPv6 network.

Commands to run yourself

The real diagnostic commands for this lab, copy-paste ready. Run these in the same order to reproduce the investigation on your own session.

Inspect IPv6 interface and RA status
show ipv6 interface GigabitEthernet0/1
Inspect IPv6 routing
show ipv6 route
Inspect IPv6 neighbors
show ipv6 neighbors
IPv6 ND: default gateway failureThe host forms a SLAAC address but never learns R1 as its default router.UpstreamISP transitdefault routeR1Gi0/02001:db8:ffff::1/64HostGi0/1 LANSLAAC addressGi0/0 uplinkdefault routeGi0/12001:db8:1::/64Router advertisements on Gi0/1The host has an address. Check whether it actually learned a default router.

Frequently asked

Why does the host have an address at all if the router is misconfigured?

SLAAC address formation and default-router learning both ride on router advertisements, but they aren't the same signal. This lab is exactly the case where the prefix information got through once but the router stopped periodically re-advertising itself as a default router.

Ready to run this lab yourself?

No setup, no image sourcing. Book a session or ask for a live demo.

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