mediumMultiple Choice
300-410 Practice Question: Is troubleshooting an IPv6 over IPv4 tunnel that…
A network engineer is troubleshooting an IPv6 over IPv4 tunnel that is used to connect two remote sites. The tunnel is configured with a tunnel source that is a loopback interface. The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address. The engineer checks the routing table and sees a route to the remote loopback's IPv4 address via a default route. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the misconception that a tunnel being up guarantees end-to-end reachability, but the real issue is asymmetric routing caused by missing return routes for the tunnel source IPv4 address.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
The tunnel is up, but the engineer cannot ping the remote tunnel endpoint IPv6 address because the remote router lacks a route back to the loopback network used as the tunnel source. The remote router only has a default route, which may not cover the specific prefix of the local loopback, causing return traffic to be dropped. For IPv6 over IPv4 tunnels, the tunnel source and destination must be reachable via unicast routing; a missing or insufficient route (like a default that doesn't match) breaks bidirectional communication.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The remote router does not have a route to the loopback network used as the tunnel source; it only has a default route that may not cover that prefix.
Why this is correct
Correct because the tunnel source loopback address must be reachable from the remote router. If the default route does not include that specific prefix (e.g., due to routing policy or subnet mismatch), the tunnel cannot encapsulate packets.
- ✗
The tunnel destination is configured with the loopback address of the remote router, but the remote router's tunnel source is a different interface.
Why it's wrong here
A mismatched remote tunnel source does not break the tunnel, since the local destination must simply match the remote source for the tunnel to come up; it is up here. Asymmetric sources are valid when both ends reference the peer's actual source address.
- ✗
The tunnel interface is missing the 'tunnel mode ipv6ip' command.
Why it's wrong here
Omitting 'tunnel mode ipv6ip' would leave the tunnel in GRE mode, and the interface would not come up as an IPv6IP tunnel; the stem states the tunnel is up. That command is required only when building a manual IPv6-in-IPv4 tunnel without GRE.
- ✗
The IPv6 address on the tunnel interface is not in the same subnet as the remote tunnel IPv6 address.
Why it's wrong here
Tunnel endpoints are point-to-point and typically share a /64 or /127; a mismatched subnet would prevent the ping, but the stem's routing-table clue points elsewhere. Address-family mismatch is the correct diagnosis when the IPv4 underlay is reachable but the IPv6 overlay is not.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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