The answer is that the Teredo server IPv4 address is missing from the configuration on R1. This is the root cause because a Teredo tunnel requires the explicit `tunnel teredo server-ip` command to specify the IPv4 address of the Teredo server; without it, the router cannot perform the initial handshake to obtain its public IPv4 address and generate the correct Teredo IPv6 prefix. As a result, the tunnel interface never becomes fully operational, and the 2001:0:4136:E378::/64 prefix is never installed in R2’s routing table. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of Teredo tunnel prerequisites and the common misconfiguration of omitting the server IP. A frequent trap is assuming that specifying the tunnel source and mode alone is sufficient, but the server IP is mandatory for the client to bootstrap connectivity. Memory tip: “No server, no Teredo—the tunnel won’t go.”
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
A network using IPv6 over IPv4 Teredo tunnels is experiencing intermittent connectivity. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:0:4136:E378:8000:63BF:3C57:DD0B/128 tunnel source 192.0.2.1 tunnel mode ipv6ip teredo. Router R2 shows: R2# show ipv6 route 2001:0:4136:E378::/64 % Route not found. What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Teredo server IPv4 address is not configured on R1.
The Teredo tunnel on R1 requires a Teredo server to facilitate the initial configuration and to help the client discover its public IPv4 address and the Teredo relay. Without the 'tunnel teredo server-ip' command specifying the Teredo server's IPv4 address, R1 cannot complete the Teredo setup, leading to an incomplete or non-functional tunnel. This results in the IPv6 prefix 2001:0:4136:E378::/64 not being installed in R2's routing table, as the tunnel interface never becomes fully operational.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 Teredo server IPv4 address is not configured on R1.
Why this is correct
Without a Teredo server, the router cannot obtain a valid Teredo address or prefix.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel mode should be ipv6ip teredo relay instead of teredo.
Why it's wrong here
The mode is correct for a Teredo client.
✗
The IPv4 address of the Teredo relay is not reachable.
Why it's wrong here
The issue is with the server, not the relay.
✗
Teredo is not supported on Cisco IOS.
Why it's wrong here
Teredo is supported on some IOS versions.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between Teredo server and relay roles, leading candidates to incorrectly focus on relay reachability or mode syntax when the missing server configuration is the actual root cause.
Detailed technical explanation
How to think about this question
Teredo (RFC 4380) encapsulates IPv6 packets inside UDP over IPv4, using a server to assign a global IPv6 address based on the client's public IPv4 address and UDP port. The server also provides the address of a Teredo relay, which forwards traffic to the native IPv6 network. Without the server, the client cannot derive its IPv6 address or locate a relay, making the tunnel non-functional even if the relay is reachable.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The Teredo server IPv4 address is not configured on R1. — The Teredo tunnel on R1 requires a Teredo server to facilitate the initial configuration and to help the client discover its public IPv4 address and the Teredo relay. Without the 'tunnel teredo server-ip' command specifying the Teredo server's IPv4 address, R1 cannot complete the Teredo setup, leading to an incomplete or non-functional tunnel. This results in the IPv6 prefix 2001:0:4136:E378::/64 not being installed in R2's routing table, as the tunnel interface never becomes fully operational.
What should I do if I get this 300-410 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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Question Discussion
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