BeginnerPublished 2026-09-16
CCNA Break/Fix: IPv4 Crosses the WAN and IPv6 Does Not
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Lab 2 of 13 in CCNA Break/Fix II: Read the Symptom · ← Previous · Next →
A dual-stack static routing lab with a subtle IPv6-only failure. IPv4 reaches across the WAN from a branch PC to an HQ server and loopback, while IPv6 to the same destinations fails. The IPv6 addressing on interfaces is correct; the break lies elsewhere. Compare what IPv4 knows versus what IPv6 knows for the same four destinations and make IPv6 match the working IPv4 design.
Learning objectives
- Diagnose a dual-stack outage where IPv4 works end-to-end but IPv6 fails
- Trace host-initiated IPv6 traffic across a WAN and isolate the break point
- Compare IPv4 and IPv6 routing tables for the same destinations to distinguish reachability gaps
- Verify the next-hop selection and route specificity for branch/HQ LANs and loopbacks
- Confirm the repair from the end hosts and validate parity between IPv4 and IPv6 forwarding
Troubleshooting focus
- Start at the symptom: from the branch PC, confirm IPv4 pings succeed while IPv6 pings to the same HQ targets fail.
- Check both routers: compare show ip route vs show ipv6 route for the four intended destinations (HQ LAN, branch LAN, and each router's loopback).
- Trace IPv6 from the branch PC (traceroute6) and observe where it dies; correlate with the nearest router that lacks a matching IPv6 route.
- Validate that interface IPv6 addresses and link-local scope are correct on the WAN; then focus on control-plane routes rather than interface state.
- After the fix, re-run host-based IPv6 pings and traceroutes to ensure end-to-end success matches IPv4 behaviour.
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How this lab is graded
- Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with
access-class, are stated in the guide and do have to match. - Addresses, modes and protocol keywords are exact. An IP address, a subnet mask,
switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide. - Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the
running-config— a change that only exists in a terminal session never reaches the grader. - You can submit as many times as you like. Your best score stands, and each attempt tells you which checks passed so you can work the gaps.
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