Question 1,909 of 2,152

IPv6 Traffic Filtering and uRPF →hardMultiple ChoiceObjective-mapped

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.”

300-410 IPv6 Traffic Filtering and uRPF Practice Question

This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.

Question 1hardmultiple choice

Review the full OSPF breakdown →

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 →

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.

uRPF strict mode checks the source address of incoming packets against the routing table. For a router that is an ABR, some routes may be learned via OSPFv3 inter-area or external routes. If the routing table has multiple paths, the reverse path may not be via the incoming interface. Additionally, OSPFv3 uses link-local addresses for adjacencies, which are not affected because uRPF is not applied to OSPFv3 packets if the router uses 'ipv6 uRPF' but OSPFv3 packets are sourced from link-local addresses that are not in the routing table, but the engineer may have configured 'allow-default' or not. The edge case: uRPF strict mode can drop traffic that arrives on a different interface than the best reverse path, especially if the router has multiple interfaces in the same area or different areas. However, a more subtle issue: if the router has a default route via another interface, uRPF strict mode without 'allow-default' will drop packets whose source address matches the default route because the default route points to 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 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
OSPF neighbours must agree on key parameters.
✗
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: 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 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. — uRPF strict mode checks the source address of incoming packets against the routing table. For a router that is an ABR, some routes may be learned via OSPFv3 inter-area or external routes. If the routing table has multiple paths, the reverse path may not be via the incoming interface. Additionally, OSPFv3 uses link-local addresses for adjacencies, which are not affected because uRPF is not applied to OSPFv3 packets if the router uses 'ipv6 uRPF' but OSPFv3 packets are sourced from link-local addresses that are not in the routing table, but the engineer may have configured 'allow-default' or not. The edge case: uRPF strict mode can drop traffic that arrives on a different interface than the best reverse path, especially if the router has multiple interfaces in the same area or different areas. However, a more subtle issue: if the router has a default route via another interface, uRPF strict mode without 'allow-default' will drop packets whose source address matches the default route because the default route points to a different interface.

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

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?

OSPF neighbours must agree on key parameters.

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