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300-410 An OSPFv3 network has multiple areas Practice Question
An OSPFv3 network has multiple areas. Area 0 includes R1 and R2. Area 1 includes R2 and R3. R2 is an ABR. R1 has an IPv6 ACL applied inbound on the interface to R2 that permits only OSPFv3 and denies all other traffic. R3 advertises a prefix 2001:db8:3::/48 into Area 1. R1's routing table shows the prefix but with a next-hop of R2. R1's uRPF is configured in strict mode on the interface to R2. Traffic from R1 to 2001:db8:3::1 is dropped. R1 shows 'show ipv6 cef 2001:db8:3::/48' points to R2's link-local address. What is the root cause?
⚠ Common exam trap
Cisco often tests the distinction between uRPF dropping forward traffic due to source address validation versus ACLs blocking return traffic, leading candidates to incorrectly blame the ACL when the real issue is the uRPF check failing on the source address.
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 source address of the traffic from R1 is not in the FIB as reachable via the interface facing R2, causing uRPF strict mode to drop the packet.
The root cause is that R1's uRPF strict mode checks the source address of incoming packets against the FIB to ensure the best reverse path to that source is via the same interface. Traffic from R1 to 2001:db8:3::1 uses a source address from R1's own subnet, but the FIB entry for that source address points out a different interface (or is not present), causing uRPF to drop the packet. The ACL permits OSPFv3 but blocks all other traffic, which prevents the return traffic from R3 from reaching R1, but the immediate failure is due to uRPF dropping the forward packet.
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 source address of the traffic from R1 is not in the FIB as reachable via the interface facing R2, causing uRPF strict mode to drop the packet.
Why this is correct
uRPF strict requires the source address to be reachable via the incoming interface. If the source is a directly connected subnet that is not in the FIB (e.g., due to filtering), the check fails.
- ✗
The ACL on R1 blocks the return traffic from the destination, causing asymmetric routing.
Why it's wrong here
The ACL permits OSPFv3, but data traffic is blocked. However, uRPF is the mechanism dropping the traffic.
- ✗
R2's link-local address is not reachable due to OSPFv3 network type mismatch.
Why it's wrong here
The prefix is installed with a next-hop, so reachability is fine.
- ✗
The prefix 2001:db8:3::/48 is not in the OSPFv3 database on R1 due to area filtering.
Why it's wrong here
The prefix is in the routing table, so it is in the database.
Visual reference
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
Introduction to ENARSI Exam and Network Fundamentals
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.
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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