This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 engineer runs the following command on Router R1:
R1# show ipv6 interface gigabitethernet 0/0 | include uRPF
IPv6 uRPF: strict mode (allow default route)
Based on this output, what is the uRPF configuration on this interface?
uRPF is disabled
Why wrong: The output shows uRPF is enabled.
uRPF is enabled in strict mode with allow-default
The output confirms strict mode uRPF with the allow-default option.
uRPF is enabled in loose mode
Why wrong: The output specifies strict mode.
uRPF is enabled but only for multicast
Why wrong: uRPF applies to unicast traffic.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
uRPF is enabled in strict mode with allow-default
The command output explicitly shows 'IPv6 uRPF: strict mode (allow default route)', which confirms that uRPF is enabled in strict mode with the allow-default option. This configuration validates that the source address of incoming packets must have a matching route in the FIB pointing back to the same interface, and the allow-default keyword permits the use of a default route as a valid reverse path, which is necessary in scenarios where a default route is the only route to the source.
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.
uRPF is disabled
Why it's wrong here
The output shows uRPF is enabled.
uRPF is enabled in strict mode with allow-default
Why this is correct
The output confirms strict mode uRPF with the allow-default option.
Related concept
Read the scenario before looking for a memorised answer.
uRPF is enabled in loose mode
Why it's wrong here
The output specifies strict mode.
uRPF is enabled but only for multicast
Why it's wrong here
uRPF applies to unicast traffic.
Common exam traps
Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates may overlook the 'allow default route' phrase and incorrectly assume that strict mode always drops packets matching a default route, or they may confuse strict mode with loose mode when the output explicitly states 'strict'.
Trap categories for this question
Command / output trap
The output shows uRPF is enabled.
Detailed technical explanation
The allow-default option is critical in networks where a default route is the only path to reach certain source addresses, such as in stub networks or when using a single upstream provider. Without allow-default, strict mode would drop packets whose source address is reachable only via a default route, because the reverse path check would fail. This configuration is often used in conjunction with BGP to prevent spoofing while accommodating default routing.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
What to study next
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FAQ
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
The correct answer is: uRPF is enabled in strict mode with allow-default — The command output explicitly shows 'IPv6 uRPF: strict mode (allow default route)', which confirms that uRPF is enabled in strict mode with the allow-default option. This configuration validates that the source address of incoming packets must have a matching route in the FIB pointing back to the same interface, and the allow-default keyword permits the use of a default route as a valid reverse path, which is necessary in scenarios where a default route is the only route to the source.
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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