- A
Apply uRPF loose mode on the customer-facing interface.
Correct. uRPF loose mode drops packets if the source address is not in the FIB, which helps prevent spoofing.
- B
Configure an inbound IPv6 ACL on the edge interface that denies traffic from reserved or internal source prefixes.
Correct. This blocks traffic with obviously spoofed source addresses (e.g., from internal ranges or reserved addresses).
- C
Use the 'ipv6 verify unicast source reachable-via any allow-default' command to allow traffic with default route.
Why wrong: Incorrect. The 'allow-default' option permits traffic even if the source is only reachable via a default route, which reduces spoofing protection.
- D
Apply an outbound IPv6 ACL on the BGP neighbor interface to filter spoofed source addresses.
Why wrong: Incorrect. Outbound ACLs filter traffic leaving the router, not incoming spoofed traffic. Spoofed traffic enters the network, so inbound filtering is needed.
- E
Disable BGP prefix filtering to allow all prefixes, then rely on uRPF only.
Why wrong: Incorrect. Disabling prefix filtering would allow potentially malicious prefixes to be installed, making uRPF less effective. Combining both is better.
Quick Answer
The correct answer is to configure an inbound IPv6 ACL on the edge interface that denies traffic from reserved or internal source prefixes, paired with enabling unicast Reverse Path Forwarding (uRPF) on the same interface. These two actions work together to prevent IPv6 spoofing by first filtering out obviously illegitimate source addresses—such as link-local, multicast, or internal prefixes—at the network edge, while uRPF verifies that the source address of incoming packets has a valid return path in the routing table, dropping any traffic that fails this check. On the Cisco CCNP ENARSI 300-410 exam, this topic tests your understanding of IPv6 spoofing mitigation techniques within a BGP-routed network, where attackers may try to inject traffic with forged source addresses to bypass security controls. A common trap is to rely solely on BGP prefix filtering, which only controls route advertisements, not the forwarding of spoofed packets. Remember the mnemonic “ACL then uRPF” to recall that ACLs block obviously bad addresses first, while uRPF validates the routing path—together they lock the door and check the ID.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which TWO actions will prevent IPv6 traffic with a spoofed source address from being forwarded in a network where BGP is used for routing? (Choose TWO.)
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
Apply uRPF loose mode on the customer-facing interface.
To prevent spoofed IPv6 traffic, you can use uRPF on edge interfaces to verify source addresses. Additionally, you can use BGP prefix filtering to ensure that only legitimate prefixes are advertised, reducing the chance of spoofed traffic being routed. Another method is to use an inbound IPv6 ACL on the edge interface to block traffic from private or reserved source addresses. The question asks for two actions among the options.
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.
- ✓
Apply uRPF loose mode on the customer-facing interface.
Why this is correct
Correct. uRPF loose mode drops packets if the source address is not in the FIB, which helps prevent spoofing.
Related concept
OSPF neighbours must agree on key parameters.
- ✓
Configure an inbound IPv6 ACL on the edge interface that denies traffic from reserved or internal source prefixes.
Why this is correct
Correct. This blocks traffic with obviously spoofed source addresses (e.g., from internal ranges or reserved addresses).
Related concept
OSPF neighbours must agree on key parameters.
- ✗
Use the 'ipv6 verify unicast source reachable-via any allow-default' command to allow traffic with default route.
Why it's wrong here
Incorrect. The 'allow-default' option permits traffic even if the source is only reachable via a default route, which reduces spoofing protection.
- ✗
Apply an outbound IPv6 ACL on the BGP neighbor interface to filter spoofed source addresses.
Why it's wrong here
Incorrect. Outbound ACLs filter traffic leaving the router, not incoming spoofed traffic. Spoofed traffic enters the network, so inbound filtering is needed.
- ✗
Disable BGP prefix filtering to allow all prefixes, then rely on uRPF only.
Why it's wrong here
Incorrect. Disabling prefix filtering would allow potentially malicious prefixes to be installed, making uRPF less effective. Combining both is better.
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: Apply uRPF loose mode on the customer-facing interface. — To prevent spoofed IPv6 traffic, you can use uRPF on edge interfaces to verify source addresses. Additionally, you can use BGP prefix filtering to ensure that only legitimate prefixes are advertised, reducing the chance of spoofed traffic being routed. Another method is to use an inbound IPv6 ACL on the edge interface to block traffic from private or reserved source addresses. The question asks for two actions among the options.
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.
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Last reviewed: Jun 18, 2026
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.
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