- A
The source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.
Correct. Link-local addresses are not in the routing table, so uRPF loose mode drops them.
- B
The router has 'ipv6 uRPF strict' configured instead of loose, but the show command indicates loose.
Why wrong: Incorrect. The configuration is loose mode.
- C
The equal-cost paths cause the routing table to have multiple entries, and uRPF loose mode requires a single best path.
Why wrong: Incorrect. uRPF loose mode works with multiple paths; it just needs any entry.
- D
The interface has an IPv6 ACL that denies the traffic before uRPF is applied.
Why wrong: Incorrect. ACLs are applied before uRPF, but the question implies uRPF is the cause.
Quick Answer
The answer is that uRPF loose mode drops the traffic because the source address is a link-local address (fe80::), which is not installed in the global routing table. Loose mode only verifies that a matching route exists for the source in the routing table, without caring which interface it arrived on, so if the source is reachable via an equal-cost path on another interface, the check should normally pass. However, link-local addresses are never placed in the global IPv6 routing table—they are only used for neighbor discovery and local link operations—so uRPF loose mode finds no entry and drops the packet, even though the source is technically reachable via a different path. This scenario tests your understanding of how uRPF interacts with IPv6 addressing on the CCNP ENARSI 300-410 exam, where the common trap is assuming that a reachable source automatically has a routing table entry. Remember the tip: “Link-local, no table—loose mode drops the cable.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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 uRPF loose mode on an interface that is part of an IPv6 network with multiple equal-cost paths to the same destination via different interfaces. The router receives traffic from a source that is reachable via one of the paths, but the traffic arrives on a different interface. The traffic 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.
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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.
uRPF loose mode checks that the source address has a routing table entry (any interface), but it does not require the incoming interface to match the reverse path. However, if the source address is not in the routing table at all, loose mode drops the packet. In this scenario, the source address is reachable via one interface, so it should be in the routing table. The edge case: if the routing table has the source prefix but with a different prefix length (e.g., a more specific route), the check may fail. But the most common issue: uRPF loose mode with multiple equal-cost paths still requires that the source address be present in the routing table. If the router has multiple paths, the source address is present. The traffic should not be dropped. However, a subtle corner case: if the router uses 'ipv6 uRPF loose allow-default' and the source address is only matched by a default route, it is permitted. But without 'allow-default', if the source address is only matched by a default route, it is dropped. The question says the source is reachable via one of the paths, so it is not a default route. The most likely explanation: the router has 'ipv6 uRPF loose' but the source address is a link-local address, which is not in the routing table, causing the drop.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet.
Why this is correct
Correct. Link-local addresses are not in the routing table, so uRPF loose mode drops them.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The router has 'ipv6 uRPF strict' configured instead of loose, but the show command indicates loose.
Why it's wrong here
Incorrect. The configuration is loose mode.
- ✗
The equal-cost paths cause the routing table to have multiple entries, and uRPF loose mode requires a single best path.
Why it's wrong here
Incorrect. uRPF loose mode works with multiple paths; it just needs any entry.
- ✗
The interface has an IPv6 ACL that denies the traffic before uRPF is applied.
Why it's wrong here
Incorrect. ACLs are applied before uRPF, but the question implies uRPF is the cause.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
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
Got this wrong? Here's your next step.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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IPv6 Traffic Filtering and uRPF — study guide chapter
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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 — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The source address is a link-local address (fe80::), which is not installed in the global routing table, causing uRPF loose mode to drop the packet. — uRPF loose mode checks that the source address has a routing table entry (any interface), but it does not require the incoming interface to match the reverse path. However, if the source address is not in the routing table at all, loose mode drops the packet. In this scenario, the source address is reachable via one interface, so it should be in the routing table. The edge case: if the routing table has the source prefix but with a different prefix length (e.g., a more specific route), the check may fail. But the most common issue: uRPF loose mode with multiple equal-cost paths still requires that the source address be present in the routing table. If the router has multiple paths, the source address is present. The traffic should not be dropped. However, a subtle corner case: if the router uses 'ipv6 uRPF loose allow-default' and the source address is only matched by a default route, it is permitted. But without 'allow-default', if the source address is only matched by a default route, it is dropped. The question says the source is reachable via one of the paths, so it is not a default route. The most likely explanation: the router has 'ipv6 uRPF loose' but the source address is a link-local address, which is not in the routing table, causing the drop.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
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