mediumMultiple ChoiceObjective-mapped
IPv6 uRPF Loose Mode Configuration
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?
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.”
⚠ Common exam trap
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.
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.
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.
- ✗
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.
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Same concept, more angles
1 more way 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.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.