Question 623 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Loose Mode in DMVPN Spoke — IPv6 Tunnel Address

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

An engineer configures IPv6 uRPF loose mode on an interface that connects to a DMVPN spoke. The spoke router uses NHRP to register with the hub and establishes a tunnel. Traffic from the spoke to destinations behind the hub is dropped. Which is the most likely explanation?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.
Correct. uRPF loose mode requires that the source address be present in the routing table (any interface). If the address is only in NHRP cache, not in the routing table, packets are dropped.
B
The hub has uRPF strict mode configured, which breaks the DMVPN tunnel because of asymmetric routing.
Why wrong: Incorrect. The question specifies uRPF loose mode on the spoke interface, not strict mode on the hub.
C
The spoke's NHRP registration packets are filtered by the uRPF check because they use multicast destination.
Why wrong: Incorrect. NHRP uses unicast or multicast; uRPF checks source address, not destination.
D
The tunnel interface has an IPv6 ACL that denies traffic from the spoke's tunnel address, overriding uRPF.
Why wrong: Incorrect. There is no mention of an ACL; the issue is uRPF behavior.

Full breakdown with real-world context →

Quick Answer

The answer is that the spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address. In uRPF loose mode, the router verifies that the source address of an incoming packet has at least one routing table entry, regardless of the interface; however, if the spoke’s tunnel IPv6 address is learned exclusively through NHRP and not injected into the routing table via a dynamic protocol like OSPFv3 or EIGRP, that source address will appear unreachable, and the packet is discarded. This scenario is a classic trap on the Cisco CCNP ENARSI 300-410 exam, testing your understanding of how uRPF interacts with DMVPN’s reliance on NHRP for address resolution rather than traditional routing. A common memory tip is “NHRP is not a routing protocol”—just because the hub knows the spoke’s tunnel address via NHRP does not mean the spoke’s own router has a route to that address, so uRPF loose mode sees it as a source with no valid path and drops the traffic.

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 spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.

In IPv6 uRPF loose mode, the router checks that the source address of an incoming packet is present in the routing table (any route, not necessarily via the incoming interface). On a DMVPN spoke, the tunnel IPv6 address is typically learned only via NHRP and is not installed in the global IPv6 routing table. Therefore, when the spoke sends traffic sourced from its tunnel address, uRPF loose mode drops the packet because the source address is not found in the routing table.

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 spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address.
Why this is correct
Correct. uRPF loose mode requires that the source address be present in the routing table (any interface). If the address is only in NHRP cache, not in the routing table, packets are dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The hub has uRPF strict mode configured, which breaks the DMVPN tunnel because of asymmetric routing.
Why it's wrong here
Incorrect. The question specifies uRPF loose mode on the spoke interface, not strict mode on the hub.
✗
The spoke's NHRP registration packets are filtered by the uRPF check because they use multicast destination.
Why it's wrong here
Incorrect. NHRP uses unicast or multicast; uRPF checks source address, not destination.
✗
The tunnel interface has an IPv6 ACL that denies traffic from the spoke's tunnel address, overriding uRPF.
Why it's wrong here
Incorrect. There is no mention of an ACL; the issue is uRPF behavior.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between uRPF loose and strict modes, and the trap here is that candidates assume loose mode only checks the RIB for any route, but they forget that NHRP-learned addresses are not installed in the global routing table, causing the drop.

Detailed technical explanation

How to think about this question

IPv6 uRPF loose mode only requires the source address to be reachable via any route in the FIB, not necessarily through the receiving interface. On DMVPN spokes, the tunnel IPv6 address is often a non-global address (e.g., link-local or a unique local address) or is learned dynamically via NHRP and not installed in the global routing table. This behavior is documented in RFC 3704, and Cisco implements uRPF for IPv6 using the 'ipv6 verify unicast source reachable-via any' command. In real-world deployments, this issue is commonly resolved by either using a global unicast address on the tunnel or by adding a static route for the tunnel source.

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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.

What to study next

Got this wrong? Here's your next step.

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The spoke's tunnel IPv6 address is not in the global routing table because it is only known via NHRP, causing uRPF loose mode to drop packets sourced from that address. — In IPv6 uRPF loose mode, the router checks that the source address of an incoming packet is present in the routing table (any route, not necessarily via the incoming interface). On a DMVPN spoke, the tunnel IPv6 address is typically learned only via NHRP and is not installed in the global IPv6 routing table. Therefore, when the spoke sends traffic sourced from its tunnel address, uRPF loose mode drops the packet because the source address is not found in the routing table.

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.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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