- 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.
Preventing IPv6 Spoofing
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.)
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.
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.
Option A is correct because uRPF (Unicast Reverse Path Forwarding) loose mode checks the source address of incoming IPv6 traffic against the routing table to ensure a route exists back to that source, without requiring the exact interface. This prevents spoofed source addresses from being forwarded when no route exists for the claimed source. Option B is correct because an inbound IPv6 ACL on the edge interface can explicitly deny traffic from reserved prefixes (e.g., RFC 6890) or internal prefixes that should never appear as source addresses from outside, effectively blocking spoofed traffic at the ingress point.
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.
- ✓
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
Read the scenario before looking for a memorised answer.
- ✓
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
Read the scenario before looking for a memorised answer.
- ✗
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: answer the scenario, not the keyword
Cisco often tests the distinction between uRPF loose mode and strict mode, and the trap here is that candidates may think 'ipv6 verify unicast source reachable-via any allow-default' is a valid anti-spoofing measure, when it actually permits traffic with a source reachable via the default route, undermining the purpose of source address validation.
Detailed technical explanation
How to think about this question
uRPF loose mode leverages the Forwarding Information Base (FIB) to verify reachability of the source address; if a route exists (even via a different interface), the packet is forwarded, which means it cannot block spoofed addresses that match legitimate prefixes (e.g., a spoofed internal address). Inbound ACLs on edge interfaces are a first-line defense, often combined with uRPF strict mode on customer-facing interfaces for stronger anti-spoofing, but strict mode requires the source to be reachable via the exact incoming interface, which may not be feasible in asymmetric routing scenarios. Real-world deployments often use a combination of uRPF loose mode and ACLs to filter bogon prefixes (e.g., RFC 3849 documentation prefixes, unique local addresses) at the network edge.
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
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: Apply uRPF loose mode on the customer-facing interface. — Option A is correct because uRPF (Unicast Reverse Path Forwarding) loose mode checks the source address of incoming IPv6 traffic against the routing table to ensure a route exists back to that source, without requiring the exact interface. This prevents spoofed source addresses from being forwarded when no route exists for the claimed source. Option B is correct because an inbound IPv6 ACL on the edge interface can explicitly deny traffic from reserved prefixes (e.g., RFC 6890) or internal prefixes that should never appear as source addresses from outside, effectively blocking spoofed traffic at the ingress point.
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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Last reviewed: Jul 4, 2026
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