Question 1,909 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

uRPF Strict Mode Drops Data Traffic on ABR — Asymmetric Routing

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 engineer configures IPv6 uRPF strict mode on an interface that is used for both IPv6 traffic and OSPFv3 routing. The router is an ABR with multiple areas. OSPFv3 adjacencies form correctly, but some IPv6 data traffic is dropped. The show ipv6 interface command shows uRPF is enabled. Which is the most likely explanation?

Clue words in this question

Noticing these words before you look at the options changes how you read each choice.

Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

A
The router has a default route pointing to a different interface, and uRPF strict mode without 'allow-default' drops packets whose source address is reachable via the default route.
Correct. uRPF strict mode checks the specific route, not the default. If the source address is only matched by a default route via another interface, the packet is dropped.
B
OSPFv3 adjacencies use link-local addresses, which are not checked by uRPF, but data traffic uses global addresses that are incorrectly filtered by the OSPFv3 process.
Why wrong: Incorrect. OSPFv3 does not filter data traffic; uRPF does.
C
The router has 'ipv6 uRPF allow-default' configured, but the default route is not installed, causing all traffic to be dropped.
Why wrong: Incorrect. 'allow-default' permits traffic matching the default route; without it, traffic is dropped.
D
The interface has an IPv6 ACL that denies traffic from certain prefixes, overriding uRPF.
Why wrong: Incorrect. ACLs and uRPF are independent; the question states uRPF is the cause.

Full breakdown with real-world context →

Quick Answer

The answer is that uRPF strict mode without the 'allow-default' keyword drops data traffic on an ABR when a default route points to a different interface. This occurs because uRPF strict mode performs a reverse path lookup on the source IP of incoming packets, and if the best matching route—including a default route—points out a different interface than the one the packet arrived on, the packet is discarded. On an ABR with asymmetric routing, OSPFv3 adjacencies remain intact since they use link-local addresses unaffected by uRPF, but data traffic sourced from addresses reachable via a default route fails the check. In the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how uRPF interacts with default routes and multi-area OSPFv3 topologies. A common trap is assuming OSPFv3 neighbor formation implies all traffic will pass, but uRPF strict mode only drops data packets, not routing protocol packets. Memory tip: “Default route, different route—without allow-default, packets get the boot.”

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 router has a default route pointing to a different interface, and uRPF strict mode without 'allow-default' drops packets whose source address is reachable via the default route.

Option A is correct because uRPF strict mode on an interface checks that the source address of incoming packets is reachable via the same interface. If the router has a default route pointing to a different interface, packets sourced from addresses that are only reachable via that default route will fail the RPF check and be dropped. The 'allow-default' keyword is required to exempt packets whose source is reachable via a default route from this check.

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 router has a default route pointing to a different interface, and uRPF strict mode without 'allow-default' drops packets whose source address is reachable via the default route.
Why this is correct
Correct. uRPF strict mode checks the specific route, not the default. If the source address is only matched by a default route via another interface, the packet is dropped.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
OSPFv3 adjacencies use link-local addresses, which are not checked by uRPF, but data traffic uses global addresses that are incorrectly filtered by the OSPFv3 process.
Why it's wrong here
Incorrect. OSPFv3 does not filter data traffic; uRPF does.
✗
The router has 'ipv6 uRPF allow-default' configured, but the default route is not installed, causing all traffic to be dropped.
Why it's wrong here
Incorrect. 'allow-default' permits traffic matching the default route; without it, traffic is dropped.
✗
The interface has an IPv6 ACL that denies traffic from certain prefixes, overriding uRPF.
Why it's wrong here
Incorrect. ACLs and uRPF are independent; the question states uRPF is the cause.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the nuance that uRPF strict mode drops traffic when a default route points out a different interface, and candidates forget that the 'allow-default' keyword is necessary to permit such traffic.

Detailed technical explanation

How to think about this question

Unicast Reverse Path Forwarding (uRPF) in strict mode performs a Forwarding Information Base (FIB) lookup on the source address of incoming packets; if the best return path to that source is not via the receiving interface, the packet is dropped. This is critical in asymmetric routing scenarios, such as an ABR with multiple areas where a default route may be learned via a different interface, causing legitimate traffic to be discarded unless 'allow-default' is configured.

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.

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

Got this wrong? Here's your next step.

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

Related 300-410 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Layer 3 Technologies practice questions

Practise 300-410 questions linked to Layer 3 Technologies.

EIGRP Troubleshooting practice questions

Practise 300-410 questions linked to EIGRP Troubleshooting.

OSPF Troubleshooting (v2/v3) practice questions

Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).

BGP Troubleshooting practice questions

Practise 300-410 questions linked to BGP Troubleshooting.

Route Redistribution practice questions

Practise 300-410 questions linked to Route Redistribution.

Policy-Based Routing (PBR) practice questions

Practise 300-410 questions linked to Policy-Based Routing (PBR).

VRF-Lite practice questions

Practise 300-410 questions linked to VRF-Lite.

Route Maps and Route Filtering practice questions

Practise 300-410 questions linked to Route Maps and Route Filtering.

Administrative Distance practice questions

Practise 300-410 questions linked to Administrative Distance.

Route Summarization practice questions

Practise 300-410 questions linked to Route Summarization.

Bidirectional Forwarding Detection (BFD) practice questions

Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).

VPN Technologies practice questions

Practise 300-410 questions linked to VPN Technologies.

Practice this exam

Start a free 300-410 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

300-410 practice-test guide →Study guide →Browse all practice tests

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 router has a default route pointing to a different interface, and uRPF strict mode without 'allow-default' drops packets whose source address is reachable via the default route. — Option A is correct because uRPF strict mode on an interface checks that the source address of incoming packets is reachable via the same interface. If the router has a default route pointing to a different interface, packets sourced from addresses that are only reachable via that default route will fail the RPF check and be dropped. The 'allow-default' keyword is required to exempt packets whose source is reachable via a default route from this check.

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.

Are there clue words in this question I should notice?

Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

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.