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 tunnel 0
Tunnel0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::1 Global unicast address(es): 2001:DB8:2::1, subnet is 2001:DB8:2::/64 Joined group address(es): FF02::1 FF02::2 ICMP redirects are enabled ICMP unreachables are enabled ND DAD is enabled, number of DAD attempts: 1 ND reachable time is 30000 milliseconds IPv6 uRPF: loose 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
Why wrong: The output specifies loose mode, not strict.
uRPF is enabled in loose mode
The output confirms loose mode uRPF.
uRPF is enabled but only for IPv4
Why wrong: This is an IPv6 interface, and uRPF is configured for IPv6.
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 loose mode
The output explicitly shows 'IPv6 uRPF: loose mode (allow default route)', which confirms that unicast Reverse Path Forwarding (uRPF) is enabled in loose mode. In loose mode, the router checks that the source address of an incoming packet has a matching entry in the routing table, but it does not require the incoming interface to match the best return path. This is distinct from strict mode, which requires both a routing table entry and that the incoming interface is the same as the outgoing interface for the return route.
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
Why it's wrong here
The output specifies loose mode, not strict.
uRPF is enabled in loose mode
Why this is correct
The output confirms loose mode uRPF.
Related concept
Read the scenario before looking for a memorised answer.
uRPF is enabled but only for IPv4
Why it's wrong here
This is an IPv6 interface, and uRPF is configured for IPv6.
Common exam traps
Cisco often tests the distinction between uRPF strict and loose modes by showing output that includes 'loose mode' or 'allow default route', and the trap here is that candidates may confuse 'loose mode' with 'disabled' or incorrectly assume that uRPF only applies to IPv4, ignoring the IPv6-specific output.
Trap categories for this question
Command / output trap
The output shows uRPF is enabled.
Detailed technical explanation
uRPF loose mode is often used in scenarios where asymmetric routing is present, such as in ISP edge networks or with multihomed connections, because it only verifies that the source address is reachable via any route (including a default route) without requiring interface symmetry. The 'allow default route' keyword in the output indicates that a default route (::/0) is considered a valid path for source address verification, which is a common configuration for networks relying on default routes for external connectivity. This behavior is defined in RFC 3704, which distinguishes between strict and loose modes for ingress filtering.
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 loose mode — The output explicitly shows 'IPv6 uRPF: loose mode (allow default route)', which confirms that unicast Reverse Path Forwarding (uRPF) is enabled in loose mode. In loose mode, the router checks that the source address of an incoming packet has a matching entry in the routing table, but it does not require the incoming interface to match the best return path. This is distinct from strict mode, which requires both a routing table entry and that the incoming interface is the same as the outgoing interface for the return route.
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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