- A
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.
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.
- B
The ACL on R1 blocks the return traffic from the destination, causing asymmetric routing.
Why wrong: The ACL permits OSPFv3, but data traffic is blocked. However, uRPF is the mechanism dropping the traffic.
- C
R2's link-local address is not reachable due to OSPFv3 network type mismatch.
Why wrong: The prefix is installed with a next-hop, so reachability is fine.
- D
The prefix 2001:db8:3::/48 is not in the OSPFv3 database on R1 due to area filtering.
Why wrong: The prefix is in the routing table, so it is in the database.
Quick Answer
The answer is that uRPF strict mode drops the traffic because the source address of the packet from R1 is not in the FIB as reachable via the incoming interface toward R2. In strict mode, uRPF verifies that the source IP of a received packet matches a route in the FIB whose next-hop is reachable out of the exact interface where the packet arrived. Here, R1’s own source subnet may be reachable via a different interface or a summary route, so the FIB entry for that source points elsewhere—or is absent—causing the strict check to fail and the packet to be dropped. This scenario tests your understanding of how uRPF interacts with OSPFv3 and IPv6 ACLs in a multi-area design, a common trap on the Cisco CCNP ENARSI 300-410 exam where candidates mistakenly blame the ACL or OSPF routing. Remember the key: strict mode demands a matching FIB entry with the same ingress interface, not just any route. Memory tip: “Strict checks the source’s return path—same door in, same door out.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
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?
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.
uRPF strict mode checks that the source address of incoming packets is reachable via the same interface. For traffic sourced from R1's own subnet, the return route points to R2 via the same interface. However, if the source address is not in the FIB as reachable via that interface (e.g., because of a summary or default route), uRPF drops the packet. In this case, the source address is likely a prefix that is not in the FIB or is reachable via a different interface.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
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.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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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 — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: 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. — uRPF strict mode checks that the source address of incoming packets is reachable via the same interface. For traffic sourced from R1's own subnet, the return route points to R2 via the same interface. However, if the source address is not in the FIB as reachable via that interface (e.g., because of a summary or default route), uRPF drops the packet. In this case, the source address is likely a prefix that is not in the FIB or is reachable via a different interface.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
About these practice questions
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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 correctly describes the behavior of IPv6 Unicast Reverse Path Forwarding (uRPF) in strict mode?
medium- A.It verifies that the source address is in the routing table, but does not check the incoming interface.
- B.It checks that the source address is reachable via the same interface and that the route is a connected route.
- ✓ C.It verifies that the source address is reachable via the same interface and that the route points back to that interface.
- D.It only checks that the source address is not a multicast or link-local address.
Why C: Strict mode uRPF checks that the source address of an incoming packet is reachable via the same interface it arrived on, and that the route points back to that interface.
Last reviewed: Jun 18, 2026
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