Quick Answer
The answer is 255. In a manual IPv6 over IPv4 tunnel configured with tunnel mode ipv6ip, the IPv4 header’s default TTL is set to 255 because the tunnel is treated as a directly connected virtual link, and the maximum value minimizes the risk of the encapsulated packet being discarded due to TTL expiry as it traverses the IPv4 transit network, as defined in RFC 2473. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of tunnel encapsulation behavior and often appears as a distractor where candidates mistakenly assume the TTL inherits from the inner IPv6 hop limit or defaults to a lower value like 64. A common trap is confusing the IPv4 TTL of the tunnel with the IPv6 hop limit of the encapsulated packet—they are independent. Memory tip: think of the tunnel as a single virtual hop, so the outer IPv4 TTL starts at the maximum, 255, like a fresh packet on a point-to-point link.
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
In a manual IPv6 over IPv4 tunnel (tunnel mode ipv6ip), what is the default maximum number of hops (TTL) for the IPv4 header?
64
Why wrong: 64 is a common default for many protocols but not for manual tunnels.
128
Why wrong: 128 is not the default for manual tunnels.
255
The default TTL is 255 for manual IPv6 over IPv4 tunnels.
16
Why wrong: 16 is too low and not the default.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
255
In a manual IPv6 over IPv4 tunnel (tunnel mode ipv6ip), the IPv4 header's TTL (Time to Live) defaults to 255. This is because the tunnel is treated as a directly connected virtual link, and the IPv4 TTL is set to the maximum value of 255 to minimize the chance of the tunneled packet being dropped due to TTL expiry within the transit IPv4 network. This behavior is defined in RFC 2473 and is the default for IPv6-in-IPv4 tunnels on Cisco IOS.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
64
Why it's wrong here
64 is a common default for many protocols but not for manual tunnels.
128
Why it's wrong here
128 is not the default for manual tunnels.
255
Why this is correct
The default TTL is 255 for manual IPv6 over IPv4 tunnels.
Related concept
Read the scenario before looking for a memorised answer.
16
Why it's wrong here
16 is too low and not the default.
Common exam traps
Cisco often tests the distinction between the default TTL values for IPv6 (64) and the outer IPv4 header in a manual IPv6 over IPv4 tunnel (255), leading candidates to mistakenly choose 64 or 128.
Detailed technical explanation
When a router encapsulates an IPv6 packet into an IPv4 packet for a manual tunnel, it constructs a new IPv4 header. The TTL field in this outer IPv4 header is set to 255 by default, as per Cisco's implementation, to ensure the encapsulated packet can traverse a large number of IPv4 hops without being discarded. This is particularly important in complex network topologies where the tunnel path may span multiple autonomous systems; if the TTL were lower, the packet could be dropped before reaching the tunnel destination, causing connectivity issues that are difficult to diagnose.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
What to study next
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FAQ
IPv6 Tunneling Techniques — This question tests IPv6 Tunneling Techniques — Read the scenario before looking for a memorised answer..
The correct answer is: 255 — In a manual IPv6 over IPv4 tunnel (tunnel mode ipv6ip), the IPv4 header's TTL (Time to Live) defaults to 255. This is because the tunnel is treated as a directly connected virtual link, and the IPv4 TTL is set to the maximum value of 255 to minimize the chance of the tunneled packet being dropped due to TTL expiry within the transit IPv4 network. This behavior is defined in RFC 2473 and is the default for IPv6-in-IPv4 tunnels on Cisco IOS.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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