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300-410 Practice Question: An engineer configures IPv6 uRPF strict mode on…
An engineer configures IPv6 uRPF strict mode on an interface of a router that participates in OSPFv3. The router starts dropping OSPFv3 Hello packets received on that interface, causing the OSPFv3 neighbor adjacency to fail. Which is the most likely explanation?
⚠ Common exam trap
Cisco often tests the misconception that uRPF drops multicast traffic by default, but the real issue is that OSPFv3's link-local source addresses are not yet in the routing table when strict mode is applied, causing the adjacency to fail.
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 OSPFv3 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them.
IPv6 uRPF strict mode checks that the source IPv6 address of an incoming packet matches a route in the routing table and that the incoming interface is the same as the outgoing interface for that route. OSPFv3 Hello packets are sent with the link-local address of the sending interface as the source IPv6 address. If that link-local address is not present in the IPv6 routing table (e.g., because OSPFv3 has not yet installed the neighbor's link-local route), uRPF strict mode drops the packet, preventing the adjacency from forming.
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 OSPFv3 Hello packets have a source IPv6 address that is not in the routing table, causing uRPF strict mode to drop them.
Why this is correct
Correct. OSPFv3 uses link-local source addresses, which are not globally routable and not present in the routing table, so uRPF strict mode drops them.
- ✗
The OSPFv3 Hello packets are multicast to ff02::5, and uRPF strict mode drops all multicast traffic by default.
Why it's wrong here
Incorrect. uRPF strict mode does not specifically drop multicast traffic; it checks source address reachability regardless of destination.
- ✗
The OSPFv3 Hello packets have a hop limit of 1, and uRPF strict mode requires a hop limit of at least 2.
Why it's wrong here
Incorrect. uRPF does not inspect hop limit; it only verifies source address reachability.
- ✗
The interface has IPv6 unicast-routing disabled, which prevents uRPF from functioning correctly.
Why it's wrong here
Incorrect. IPv6 unicast-routing must be enabled globally, but this alone does not cause uRPF to drop OSPFv3 Hellos.
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
OSPF Neighbor Adjacency and Basics
Key term
IPv6 Routing OSPFv3
OSPFv3 is a routing protocol that lets routers share information about how to reach destinations in an IPv6 network, updating routes automatically as the network changes.
Key term
uRPF
Unicast Reverse Path Forwarding is a network security feature that verifies the source address of incoming packets to prevent IP spoofing attacks.
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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.