hardMultiple ChoiceObjective-mapped
IPv6 Unicast RPF Failure: Strict Mode and Multicast Interaction
A network engineer is troubleshooting an IPv6 multicast issue on a router. The router is receiving multicast traffic from a source, but the traffic is not being forwarded to downstream receivers. The engineer checks the interface and finds an inbound IPv6 ACL that permits only certain multicast groups. The ACL permits the group address FF1E::1, but the source address is 2001:db8:1::1. The engineer also notices that uRPF is enabled on the interface in strict mode. The router has a route to the source's network via a different interface. What is the most likely cause of the multicast forwarding failure?
Quick Answer
The answer is the uRPF strict mode check failing because the route to the source’s network points out a different interface than the one receiving the multicast packet. This occurs because strict unicast RPF verifies that the source address of an incoming IPv6 packet is reachable via the exact interface on which it arrived; if the routing table shows a different outgoing interface for that source, the packet is silently dropped before any multicast forwarding can happen. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 unicast RPF failure symptoms can masquerade as a multicast issue—a common trap is to blame the ACL or multicast configuration when the real culprit is the reverse path check. Remember that strict mode demands symmetry: the packet’s source must be reachable back out the same door it came in. A helpful memory tip is “strict checks the door, not the destination”—if the return route leaves through a different door, the packet is dropped at the doorstep.
⚠ Common exam trap
The trap here is that candidates often focus on the ACL or multicast configuration and overlook the uRPF strict mode check, which silently drops packets before multicast forwarding logic is applied.
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 uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received.
With uRPF strict mode enabled, the router checks that the source address of the incoming packet is reachable via the same interface on which the packet was received. Since the route to 2001:db8:1::1 points out a different interface, the uRPF check fails and the packet is dropped before any multicast forwarding can occur. This explains why the multicast traffic is not forwarded to downstream receivers despite the ACL permitting the group address.
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 uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received.
Why this is correct
Correct because multicast uRPF requires the source address to be reachable via the receiving interface; otherwise, the packet is dropped to prevent spoofing.
- ✗
The ACL is blocking the multicast traffic because the source address is not permitted.
Why it's wrong here
Incorrect because the ACL permits the group address, and the source address is not typically filtered in multicast ACLs unless specified.
- ✗
The multicast routing is not enabled globally.
Why it's wrong here
Incorrect because the router is receiving multicast traffic, indicating multicast routing is enabled.
- ✗
The downstream receivers are not in the multicast group.
Why it's wrong here
Incorrect because the issue is with forwarding, not group membership.
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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 THREE symptoms indicate that IPv6 unicast RPF is misconfigured or failing on an interface? (Choose THREE.)
hard- ✓ A.Traffic from a valid source IP is being dropped on the interface.
- ✓ B.The router logs 'IPv6 unicast RPF drop' messages.
- C.The command 'ipv6 verify unicast source reachable-via any' is present in the running config.
- D.High CPU utilization is observed due to RPF processing.
- ✓ E.The 'show ipv6 interface' output shows an increasing 'RPF drops' counter.
Why A: IPv6 unicast RPF (uRPF) drops traffic when the source address is not reachable via the incoming interface according to the FIB. If a valid source IP is being dropped, it indicates that the RPF check is failing, often due to asymmetric routing or a missing route in the FIB for that source prefix.
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.