Question 377 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Strict Mode with OSPFv3: Why Traffic Is Dropped

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?

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.

Full breakdown with real-world context →

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.”

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.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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
Read the scenario before looking for a memorised answer.
✗
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: answer the scenario, not the keyword

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.

Detailed technical explanation

How to think about this question

uRPF strict mode requires that the source address of any incoming packet must have a route in the FIB that points back to the same interface on which the packet arrived. In this scenario, R1's source address (e.g., from its own loopback or LAN) may not be reachable via the interface facing R2, or the FIB may have a more specific route pointing elsewhere, causing the check to fail. This is a common issue when uRPF is applied on an interface that is not the optimal reverse path for the source subnet, especially in multi-area OSPFv3 designs where the ABR may not advertise all prefixes.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

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.

What to study next

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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 — Read the scenario before looking for a memorised answer..

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. — 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.

What should I do if I get this 300-410 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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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: IPv6 uRPF strict mode requires that the source address of an incoming packet is reachable via the exact same interface on which the packet arrived, and that the best matching route in the FIB points back to that interface. This prevents spoofed traffic by ensuring the source is topologically correct from the router's perspective, as defined in RFC 3704.

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