Question 1,766 of 2,152

IPv6 Traffic Filtering and uRPF →mediumMultiple ChoiceObjective-mapped

IPv6 uRPF Loose Mode Configuration

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.

Examine this configuration:

interface GigabitEthernet0/4

ipv6 address 2001:db8:2::1/64 ipv6 verify unicast source reachable-via any

What is the effect of the 'ipv6 verify unicast source reachable-via any' command?

A
The router performs strict uRPF: the source address must be reachable via the same interface the packet arrived on.
Why wrong: The 'any' keyword indicates loose mode, not strict.
B
The router performs loose uRPF: the source address must be reachable via any route in the FIB.
Loose mode only requires a route to the source, not necessarily via the receiving interface.
C
The router drops all packets with source addresses not in the same subnet as the interface.
Why wrong: Loose mode does not require the source to be in the same subnet; it only checks for a route.
D
The command is invalid because 'ipv6 verify unicast' requires a route-map.
Why wrong: The command is valid; the 'reachable-via any' option is a standard uRPF configuration.

Full breakdown with real-world context →

Quick Answer

The correct answer is that the router performs loose uRPF, meaning the source address must be reachable via any route in the FIB. This is because the command `ipv6 verify unicast source reachable-via any` explicitly enables loose mode verification, which checks only that a valid route to the source exists somewhere in the routing table—without requiring that the incoming interface matches the reverse path interface. On the Cisco CCNP ENARSI 300-410 exam, this configuration tests your understanding of uRPF modes and their anti-spoofing behavior; a common trap is confusing loose mode with strict mode, which demands both route existence and interface match. Remember the memory tip: “Loose is lenient—any route will do; strict is specific—same interface, too.”

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 router performs loose uRPF: the source address must be reachable via any route in the FIB.

The command 'ipv6 verify unicast source reachable-via any' enables loose unicast Reverse Path Forwarding (uRPF) for IPv6. In loose mode, the router checks that the source address of an incoming packet is reachable via any route in the Forwarding Information Base (FIB), not necessarily through the receiving interface. This is the correct behavior described in option B.

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 router performs strict uRPF: the source address must be reachable via the same interface the packet arrived on.
Why it's wrong here
The 'any' keyword indicates loose mode, not strict.
✓
The router performs loose uRPF: the source address must be reachable via any route in the FIB.
Why this is correct
Loose mode only requires a route to the source, not necessarily via the receiving interface.
Related concept
Read the scenario before looking for a memorised answer.
✗
The router drops all packets with source addresses not in the same subnet as the interface.
Why it's wrong here
Loose mode does not require the source to be in the same subnet; it only checks for a route.
✗
The command is invalid because 'ipv6 verify unicast' requires a route-map.
Why it's wrong here
The command is valid; the 'reachable-via any' option is a standard uRPF configuration.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the distinction between 'any' (loose) and 'rx' (strict) keywords in uRPF configuration, and candidates commonly confuse 'reachable-via any' with strict uRPF or assume it requires a route-map.

Trap categories for this question

Keyword trap
The 'any' keyword indicates loose mode, not strict.
Command / output trap
The command is valid; the 'reachable-via any' option is a standard uRPF configuration.

Detailed technical explanation

How to think about this question

Under the hood, loose uRPF performs a FIB lookup for the source IPv6 address and only drops the packet if no route exists at all, making it useful for mitigating spoofed traffic in asymmetric routing environments where strict uRPF would drop legitimate packets. The command uses the 'any' keyword to bypass the interface-specific check, relying solely on the presence of a route in the FIB. In real-world scenarios, this is often deployed on customer-facing interfaces where return traffic may take a different path through the network.

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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.

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 router performs loose uRPF: the source address must be reachable via any route in the FIB. — The command 'ipv6 verify unicast source reachable-via any' enables loose unicast Reverse Path Forwarding (uRPF) for IPv6. In loose mode, the router checks that the source address of an incoming packet is reachable via any route in the Forwarding Information Base (FIB), not necessarily through the receiving interface. This is the correct behavior described in option B.

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 statement about IPv6 uRPF loose mode is true?

medium

A.It requires the source address to be reachable via the same interface.
✓ B.It only verifies that the source address exists in the FIB.
C.It drops packets with link-local source addresses.
D.It is enabled by default on all interfaces.

Why B: In IPv6 unicast Reverse Path Forwarding (uRPF) loose mode, the router checks the FIB (Forwarding Information Base) to verify that the source address of an incoming packet exists in the routing table. It does not require the source address to be reachable via the same interface, which is the key distinction from strict mode. This allows loose mode to be used in asymmetric routing scenarios where the return path may not match the ingress interface.

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