- A
Apply an inbound IPv6 access-list on the interface that denies traffic from the source prefix.
Correct. An inbound IPv6 ACL on the interface will drop packets from the specified source prefix before any routing decision, preventing forwarding.
- B
Configure a route map that matches the source prefix and sets a BGP community, then apply it inbound on the BGP neighbor to filter the prefix from being advertised.
Why wrong: Incorrect. A route map applied inbound on a BGP neighbor filters the routes being advertised/received, not the traffic itself. It does not prevent forwarding of packets already sourced from that prefix.
- C
Use the 'ipv6 route' command to install a discard route for the source prefix.
Why wrong: Incorrect. A discard route for the source prefix drops traffic destined to that prefix, not traffic sourced from it.
- D
Apply an outbound IPv6 access-list on the interface to block traffic from the source prefix.
Correct. An outbound IPv6 ACL on the interface will drop packets from the specified source prefix before they leave the router, preventing forwarding out that interface.
- E
Configure uRPF strict mode on the interface to drop packets from the source prefix if the prefix is not in the FIB.
Why wrong: Incorrect. uRPF strict mode drops packets if the source prefix is not in the FIB. If the source prefix is reachable, packets will pass. It does not guarantee blocking all traffic from that source.
Two Ways to Block IPv6 Traffic from a Source Prefix Without a Prefix List
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. A key principle to apply: inbound ACL. 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 configuration changes will prevent IPv6 traffic from being forwarded from a specific source prefix in a BGP environment without using a prefix list? (Choose TWO.)
Quick Answer
The answer is configuring an inbound IPv6 access-list on the interface or applying a route map that matches the source prefix and sets a denied BGP community. These two methods block IPv6 traffic from a specific source prefix without relying on a prefix list, because an access-list filters traffic at the interface level based on source addresses, while BGP community-based filtering uses a route map to tag and deny the prefix at the neighbor inbound stage, preventing its advertisement and subsequent forwarding. On the Cisco CCNP ENARSI 300-410 exam, this tests your ability to distinguish between traffic filtering and routing policy tools—a common trap is confusing route filtering (like distribute-lists) with actual traffic blocking, which requires interface ACLs or community manipulation. Remember the memory tip: “ACL for the wire, community for the peer” to recall that access-lists block traffic directly, while BGP communities block the route from being shared, indirectly stopping traffic.
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 an inbound IPv6 access-list on the interface that denies traffic from the source prefix.
Option A is correct because an inbound IPv6 ACL on the interface will drop packets from the specified source prefix as soon as they enter the router, preventing forwarding. Option D is correct because an outbound IPv6 ACL on the interface will drop packets from that source prefix just before they are forwarded out, also preventing forwarding. Option B is incorrect because configuring a route map to filter BGP advertisements only prevents the prefix from being learned or advertised by BGP; it does not drop packets already sourced from that prefix. Options C and E are also incorrect: a discard route drops traffic destined to the prefix, not sourced from it; uRPF strict mode drops packets only when the source prefix is not in the FIB, which does not guarantee blocking traffic from a prefix that is reachable.
Key principle: Inbound ACL
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 an inbound IPv6 access-list on the interface that denies traffic from the source prefix.
- ✗
Configure a route map that matches the source prefix and sets a BGP community, then apply it inbound on the BGP neighbor to filter the prefix from being advertised.
Why it's wrong here
Incorrect. A route map applied inbound on a BGP neighbor filters the routes being advertised/received, not the traffic itself. It does not prevent forwarding of packets already sourced from that prefix.
- ✗
Use the 'ipv6 route' command to install a discard route for the source prefix.
Why it's wrong here
Incorrect. A discard route for the source prefix drops traffic destined to that prefix, not traffic sourced from it.
- ✓
Apply an outbound IPv6 access-list on the interface to block traffic from the source prefix.
- ✗
Configure uRPF strict mode on the interface to drop packets from the source prefix if the prefix is not in the FIB.
Why it's wrong here
Incorrect. uRPF strict mode drops packets if the source prefix is not in the FIB. If the source prefix is reachable, packets will pass. It does not guarantee blocking all traffic from that source.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The key distinction is between filtering traffic based on source address using interface ACLs (which block actual packet forwarding) versus filtering routing updates using route maps or prefix lists (which affect route advertisements but not the forwarding of packets already present). Candidates often confuse these two mechanisms.
Detailed technical explanation
How to think about this question
IPv6 access-lists use the 'ipv6 access-list' command and can match on source and destination addresses, as well as upper-layer protocols. Inbound ACLs are processed before routing, making them effective for source-based traffic filtering without affecting BGP advertisements. This approach is commonly used in scenarios where you need to block traffic from a specific customer or network segment without altering BGP routing policies.
KKey Concepts to Remember
- Inbound ACL
- Outbound ACL
- Route filtering vs traffic filtering
- uRPF strict mode
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
Inbound ACL
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
Got this wrong? Here's your next step.
Review inbound ACL, then practise related 300-410 questions on the same topic to reinforce the concept.
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IPv6 Traffic Filtering and uRPF — study guide chapter
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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 — Inbound ACL.
What is the correct answer to this question?
The correct answer is: Apply an inbound IPv6 access-list on the interface that denies traffic from the source prefix. — Option A is correct because an inbound IPv6 ACL on the interface will drop packets from the specified source prefix as soon as they enter the router, preventing forwarding. Option D is correct because an outbound IPv6 ACL on the interface will drop packets from that source prefix just before they are forwarded out, also preventing forwarding. Option B is incorrect because configuring a route map to filter BGP advertisements only prevents the prefix from being learned or advertised by BGP; it does not drop packets already sourced from that prefix. Options C and E are also incorrect: a discard route drops traffic destined to the prefix, not sourced from it; uRPF strict mode drops packets only when the source prefix is not in the FIB, which does not guarantee blocking traffic from a prefix that is reachable.
What should I do if I get this 300-410 question wrong?
Review inbound ACL, then practise related 300-410 questions on the same topic to reinforce the concept.
What is the key concept behind this question?
Inbound ACL
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Last reviewed: Jul 4, 2026
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