The answer is that R1’s use of the eui-64 keyword on the ISATAP tunnel causes the IPv6 interface identifier to be derived from its MAC address instead of the required ISATAP format. ISATAP tunnels mandate that the interface identifier be constructed using the pattern ::5EFE:w.x.y.z, where w.x.y.z is the IPv4 tunnel source address, and R2’s global address 2001:DB8:1::5EFE:192.0.2.2 correctly follows this standard. On the Cisco CCNP ENARSI 300-410 exam, this question tests your understanding of the ISATAP interface identifier format and the common misconfiguration of using eui-64 instead of letting the tunnel mode automatically generate the correct identifier. A frequent trap is assuming eui-64 works universally, but ISATAP is an exception—it always requires the embedded IPv4 address. Remember the mnemonic: “ISATAP always maps the IPv4 source into the last 32 bits via 5EFE; never let a MAC address interfere.”
300-410 IPv6 Tunneling Techniques Practice Question
This 300-410 practice question tests your understanding of ipv6 tunneling techniques. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 ISATAP tunnels is experiencing connectivity issues. Router R1 has the following relevant configuration: interface Tunnel0 ipv6 address 2001:DB8:1::/64 eui-64 tunnel source 192.0.2.1 tunnel mode ipv6ip isatap. Router R2 shows: R2# show ipv6 interface Tunnel0 Tunnel0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::5EFE:192.0.2.2 Global unicast address(es): 2001:DB8:1::5EFE:192.0.2.2. 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 tunnel mode should be ipv6ip isatap on both ends, but R2's address shows correct ISATAP format, indicating R1's address is wrong.
Option B is correct because ISATAP tunnels require both endpoints to use the same tunnel mode (ipv6ip isatap) and to form the IPv6 interface identifier from the IPv4 tunnel source using the format ::5EFE:w.x.y.z. R2's global address 2001:DB8:1::5EFE:192.0.2.2 correctly embeds its IPv4 address 192.0.2.2, but R1's configuration with the eui-64 keyword causes it to derive an interface identifier from its MAC address instead of the required ISATAP format. This mismatch prevents proper IPv6 neighbor discovery and connectivity over the tunnel.
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 IPv6 address on R1 should be configured without the eui-64 keyword to allow manual assignment.
Why it's wrong here
The issue is the address format, not the method of assignment.
✓
The tunnel mode should be ipv6ip isatap on both ends, but R2's address shows correct ISATAP format, indicating R1's address is wrong.
Why this is correct
R1's address uses EUI-64, which does not produce the ISATAP-specific interface identifier, causing address mismatch.
Related concept
Read the scenario before looking for a memorised answer.
✗
The tunnel source on R1 should be the IPv4 address of R2.
Why it's wrong here
The tunnel source is the local IPv4 address, which is correct.
✗
ISATAP requires the use of a link-local address only.
Why it's wrong here
Global unicast addresses are also used in ISATAP.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between standard EUI-64 address generation and ISATAP's fixed interface identifier format, trapping candidates who assume that eui-64 is always correct for IPv6 tunnel interfaces.
Detailed technical explanation
How to think about this question
ISATAP (Intra-Site Automatic Tunnel Addressing Protocol) uses a modified EUI-64 format where the interface identifier is constructed as ::0:5EFE:IPv4-address for global addresses and ::0200:5EFE:IPv4-address for link-local addresses, as defined in RFC 5214. The eui-64 keyword in Cisco IOS triggers standard EUI-64 generation from the interface MAC, which is incompatible with ISATAP's required identifier format; instead, the address must be configured with a static prefix and the router automatically appends the ISATAP identifier based on the tunnel source. In real-world deployments, misconfiguration like this leads to silent packet drops because the IPv6 neighbor cache on R2 will not resolve R1's link-local address, preventing ND (Neighbor Discovery) from completing.
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 network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
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 tunnel mode should be ipv6ip isatap on both ends, but R2's address shows correct ISATAP format, indicating R1's address is wrong. — Option B is correct because ISATAP tunnels require both endpoints to use the same tunnel mode (ipv6ip isatap) and to form the IPv6 interface identifier from the IPv4 tunnel source using the format ::5EFE:w.x.y.z. R2's global address 2001:DB8:1::5EFE:192.0.2.2 correctly embeds its IPv4 address 192.0.2.2, but R1's configuration with the eui-64 keyword causes it to derive an interface identifier from its MAC address instead of the required ISATAP format. This mismatch prevents proper IPv6 neighbor discovery and connectivity over the tunnel.
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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