Question 455 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple SelectObjective-mapped

IPv6 uRPF Strict vs Loose Mode

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.

Which TWO statements about IPv6 unicast RPF (uRPF) behavior in Cisco IOS-XE are correct? (Choose TWO.)

A
Strict mode uRPF drops packets if the source address is not reachable via the incoming interface.
Correct. Strict mode requires the source prefix to be reachable via the exact incoming interface.
B
Loose mode uRPF drops packets if the source address is not present in the FIB at all.
Correct. Loose mode only checks that the source address exists in the FIB, ignoring the interface.
C
The default uRPF mode on Cisco IOS-XE is strict mode.
Why wrong: Incorrect. The default is loose mode when 'ipv6 verify unicast source reachable-via' is configured without specifying 'rx'.
D
IPv6 uRPF can be applied globally using the 'ipv6 unicast-rpf' command.
Why wrong: Incorrect. IPv6 uRPF is configured per interface using 'ipv6 verify unicast source reachable-via', not globally.
E
uRPF checks both source and destination addresses in the IPv6 header.
Why wrong: Incorrect. uRPF only verifies the source address against the FIB; destination address is not checked.

Full breakdown with real-world context →

Quick Answer

The correct answer is that loose mode uRPF drops packets if the source address is not present in the FIB at all, while strict mode additionally requires the source prefix to match the incoming interface. This distinction is critical because IPv6 uRPF verifies the source address against the Forwarding Information Base (FIB), not the destination. Strict mode, enabled with `ipv6 verify unicast source reachable-via rx`, demands both a FIB match and that the return path exits the same interface, making it vulnerable to dropping legitimate traffic under asymmetric routing. Loose mode, set with `reachable-via any`, only checks for any FIB entry for the source, ignoring the interface, which is more tolerant of asymmetric paths but still blocks packets with completely bogus source addresses. On the Cisco CCNP ENARSI 300-410 exam, this topic tests your ability to differentiate between the two modes and their configuration commands, often with a trap involving the default mode—remember, the default is loose, not strict. A simple memory tip: “Strict is strict on the interface; Loose is loose on the interface.”

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

✓

Strict mode uRPF drops packets if the source address is not reachable via the incoming interface.

Option A is correct because IPv6 unicast RPF strict mode verifies that the source address of an incoming packet is reachable via the exact interface on which the packet arrived, according to the FIB. If the source address is not reachable through that interface, the packet is dropped. This prevents spoofed traffic from entering the network from an unexpected path.

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.

✓
Strict mode uRPF drops packets if the source address is not reachable via the incoming interface.
Why this is correct
Correct. Strict mode requires the source prefix to be reachable via the exact incoming interface.
Related concept
Read the scenario before looking for a memorised answer.
✓
Loose mode uRPF drops packets if the source address is not present in the FIB at all.
Why this is correct
Correct. Loose mode only checks that the source address exists in the FIB, ignoring the interface.
Related concept
Read the scenario before looking for a memorised answer.
✗
The default uRPF mode on Cisco IOS-XE is strict mode.
Why it's wrong here
Incorrect. The default is loose mode when 'ipv6 verify unicast source reachable-via' is configured without specifying 'rx'.
✗
IPv6 uRPF can be applied globally using the 'ipv6 unicast-rpf' command.
Why it's wrong here
Incorrect. IPv6 uRPF is configured per interface using 'ipv6 verify unicast source reachable-via', not globally.
✗
uRPF checks both source and destination addresses in the IPv6 header.
Why it's wrong here
Incorrect. uRPF only verifies the source address against the FIB; destination address is not checked.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the misconception that uRPF is enabled by default or that it can be applied globally, when in fact it is an interface-level feature that must be explicitly configured per interface.

Detailed technical explanation

How to think about this question

IPv6 uRPF leverages the FIB (Forwarding Information Base) built from routing protocols or static routes. In strict mode, the router performs a reverse path lookup: it checks if the source address is reachable via the incoming interface, and if multiple equal-cost paths exist, the packet is accepted only if the incoming interface matches one of those paths. Loose mode only requires the source address to exist in the FIB, regardless of the incoming interface, which is useful for multihomed networks where asymmetric routing is expected.

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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

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: Strict mode uRPF drops packets if the source address is not reachable via the incoming interface. — Option A is correct because IPv6 unicast RPF strict mode verifies that the source address of an incoming packet is reachable via the exact interface on which the packet arrived, according to the FIB. If the source address is not reachable through that interface, the packet is dropped. This prevents spoofed traffic from entering the network from an unexpected path.

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.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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Same concept, more angles

1 more ways this is tested on 300-410

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. Which THREE statements about IPv6 unicast RPF (uRPF) are true? (Choose THREE.)

hard

✓ A.IPv6 uRPF uses the Forwarding Information Base (FIB) to verify source address reachability.
✓ B.IPv6 uRPF can be configured in strict mode or loose mode.
✓ C.IPv6 uRPF is configured on a per-interface basis.
D.IPv6 uRPF checks the destination address of incoming packets.
E.IPv6 uRPF is used to filter multicast traffic.

Why A: Option A is correct because IPv6 unicast RPF (uRPF) leverages the Forwarding Information Base (FIB) to verify that the source address of an incoming packet is reachable via the interface on which the packet was received. The FIB contains the best routes from the routing table, and uRPF performs a reverse path lookup against it to ensure the source address is topologically correct. This prevents spoofed traffic from entering the network.

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