CCNA Network Infrastructure and Connectivity Practice Question
Exhibit
Router# show ipv6 interface g0/0/0
GigabitEthernet0/0/0 is up, line protocol is up
IPv6 is enabled, link-local address is FE80::21A:2BFF:FE3C:4D5E
No global unicast address is configured
Joined group address(es):
FF02::1
FF02::2
FF02::1:FF3C:4D5E
MTU is 1500 bytes
ICMP error messages limited to one every 100 milliseconds
ICMP redirects are enabled
ICMP unreachables are sent
ND DAD is enabled, number of DAD attempts: 1
ND reachable time is 30000 milliseconds (using 30000)
ND advertised reachable time is 0 (unspecified)
ND advertised retransmit interval is 0 (unspecified)
ND router advertisements are sent every 200 seconds
ND router advertisements live for 1800 seconds
ND advertised default router preference is Medium
Hosts use stateless autoconfiguration for addresses.A network administrator is troubleshooting an IPv6 connectivity issue on a newly deployed router. The router's G0/0/0 interface is configured with an IPv6 address using EUI-64, but hosts on that subnet cannot reach the router's link-local address. The administrator runs 'show ipv6 interface g0/0/0' and sees that the interface is up/up but the IPv6 address is not in the expected format. What is the most likely cause of the problem?
⚠ Common exam trap
Cisco often tests the distinction between interface status (up/up) and configuration correctness, leading candidates to assume that a working interface means the IPv6 address is properly configured, when in fact the address may be missing or misconfigured.
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 IPv6 address was not configured correctly; the 'ipv6 address' command was likely omitted or misconfigured.
The router's G0/0/0 interface is up/up, but the IPv6 address is not in the expected EUI-64 format. This indicates that the 'ipv6 address' command was likely omitted entirely, because if it were simply misconfigured (e.g., without the `eui-64` keyword), the router would still automatically generate a link-local address, and hosts would be able to reach it. Since hosts cannot reach the link-local address, IPv6 is not enabled on the interface at all. The correct configuration requires the `ipv6 address` command with the appropriate prefix and the `eui-64` keyword.
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 interface is administratively down.
Why it's wrong here
The interface status in the output reads 'up, line protocol is up', indicating that the interface is fully operational at both Layer 1 and Layer 2. An administratively down interface would instead show 'administratively down, line protocol is down' and would not generate any IPv6 traffic, including Router Advertisements. Since the router is actively advertising and the link-local address is present, the interface is clearly not shut down or disabled.
- ✓
The IPv6 address was not configured correctly; the 'ipv6 address' command was likely omitted or misconfigured.
Why this is correct
If the 'ipv6 address' command was omitted, IPv6 is not enabled, and no link-local address exists. If it was misconfigured (e.g., missing the `eui-64` keyword), the router would still have a link-local address, so the symptom of hosts unable to reach the link-local address would not occur. Therefore, omission is the most likely cause given the symptom.
- ✗
The MAC address of the interface is invalid, preventing EUI-64 from generating a proper address.
Why it's wrong here
The link-local address FE80::21A:2BFF:FE3C:4D5E shown in the output is derived from the interface MAC address using EUI-64, which inserts FFFE in the middle and flips the U/L bit. The presence of this valid link-local address proves the MAC address is not invalid and EUI-64 generation succeeded. An invalid or all-zero MAC would prevent IPv6 from generating any link-local address, but the output clearly displays a properly formed one, so this option cannot be the cause.
- ✗
The router is not sending Router Advertisements, so hosts cannot autoconfigure.
Why it's wrong here
The output explicitly states 'ND router advertisements are sent every 200 seconds', which confirms the router is actively sending ICMPv6 Router Advertisements on this interface. Hosts would therefore be able to perform stateless address autoconfiguration (SLAAC) and learn the default router. However, the problem is that hosts cannot reach the router's own link-local address, which points to a missing or incorrectly configured IPv6 address on the router itself, not to a lack of RA messages.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓The IPv6 address was not configured correctly; the 'ipv6 address' command was likely omitted or misconfigured.Correct answer▾
Why this is correct
If the 'ipv6 address' command was omitted, IPv6 is not enabled, and no link-local address exists. If it was misconfigured (e.g., missing the `eui-64` keyword), the router would still have a link-local address, so the symptom of hosts unable to reach the link-local address would not occur. Therefore, omission is the most likely cause given the symptom.
✗The interface is administratively down.Wrong answer — click to see why▾
Why this is wrong here
The 'show ipv6 interface' output shows 'up, line protocol is up', indicating the interface is not administratively down. An administratively down interface would show 'administratively down' in the output.
Why candidates choose this
Students often confuse 'up/up' with 'administratively down' when troubleshooting connectivity issues, but the output clearly indicates the interface is operational.
✗The MAC address of the interface is invalid, preventing EUI-64 from generating a proper address.Wrong answer — click to see why▾
Why this is wrong here
The link-local address (FE80::21A:2BFF:FE3C:4D5E) is correctly formed using EUI-64, which requires a valid MAC address. The presence of 'FF:FE' in the middle indicates EUI-64 is functioning properly, so the MAC address is valid.
Why candidates choose this
Students may think that an invalid MAC address could cause EUI-64 to fail, but the output shows a properly formed link-local address, ruling out this issue.
✗The router is not sending Router Advertisements, so hosts cannot autoconfigure.Wrong answer — click to see why▾
Why this is wrong here
The output shows 'ND router advertisements are sent every 200 seconds', confirming that Router Advertisements are enabled. The problem is about the router's own IPv6 address, not host autoconfiguration.
Why candidates choose this
Router Advertisements are critical for SLAAC, and students might assume that missing RAs cause connectivity issues. However, the question focuses on the router's address, not host configuration.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
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Related to this question
Learn chapter
IPv4 Addressing and Address Classes
Key term
EUI-64
EUI-64 is a method for creating a 64-bit interface identifier from a 48-bit MAC address, used to form an IPv6 link-local or stateless address auto-configuration (SLAAC) address.
Key term
Subnet
A subnet is a logical subdivision of an IP network, created by partitioning a larger network address space using subnet masks.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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