- A
The ACL is blocking TCP packets from the neighbor's global unicast address because it only permits traffic from the link-local address.
Correct because BGP uses TCP, and the ACL permits only traffic from the link-local address, but the BGP session is established using global addresses, causing the TCP handshake to fail.
- B
The BGP update is malformed because the neighbor does not have the correct route-map applied.
Why wrong: Incorrect because the error message is likely caused by the ACL dropping packets, not a malformed update; the update is never received.
- C
The ACL is missing a permit statement for ICMPv6 neighbor discovery messages.
Why wrong: Incorrect because ND messages are not directly related to BGP TCP connections; the issue is TCP port 179.
- D
The BGP session is using link-local addresses, but the ACL permits global addresses.
Why wrong: Incorrect because the scenario states the peering uses global unicast addresses.
Quick Answer
The answer is the ACL is blocking TCP packets from the neighbor's global unicast address because it only permits traffic from the link-local address. When BGP peering is established using global unicast addresses, the TCP session’s source address is the global unicast, not the link-local, even on a point-to-point link. The inbound IPv6 ACL on R1 permits only TCP port 179 from the neighbor’s link-local address, so packets sourced from R2’s global unicast are dropped, causing the TCP connection to fail and generating the BGP Notification sent: 3/2 (Update malformed) error. This scenario tests your understanding of the critical distinction between IPv6 ACL source matching and BGP peering address types on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming link-local addresses are always used for BGP sessions over directly connected links. Remember: BGP uses the peering address as the TCP source, not the interface’s link-local. Memory tip: “Global peer, global permit—link-local ACLs will break it.”
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.
A network engineer is troubleshooting a BGP IPv6 peering issue between two routers, R1 and R2, connected via a point-to-point link. The engineer notices that the BGP session is flapping with error 'BGP Notification sent: 3/2 (Update malformed)'. The engineer checks the IPv6 ACL applied to the interface on R1 and sees an inbound ACL that permits only TCP port 179 from the neighbor's link-local address. The BGP peering uses the global unicast addresses of the interfaces. What is the most likely cause of the BGP session failure?
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.
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 ACL is blocking TCP packets from the neighbor's global unicast address because it only permits traffic from the link-local address.
The BGP session is flapping because the inbound ACL on R1 permits only TCP port 179 traffic sourced from the neighbor's link-local address, but the BGP peering is configured using global unicast addresses. Therefore, TCP packets from R2's global unicast address are dropped by the ACL, causing the BGP session to fail with a 'Notification sent: 3/2 (Update malformed)' error, as the TCP connection cannot be established or maintained.
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 ACL is blocking TCP packets from the neighbor's global unicast address because it only permits traffic from the link-local address.
Why this is correct
Correct because BGP uses TCP, and the ACL permits only traffic from the link-local address, but the BGP session is established using global addresses, causing the TCP handshake to fail.
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.
- ✗
The BGP update is malformed because the neighbor does not have the correct route-map applied.
Why it's wrong here
Incorrect because the error message is likely caused by the ACL dropping packets, not a malformed update; the update is never received.
- ✗
The ACL is missing a permit statement for ICMPv6 neighbor discovery messages.
- ✗
The BGP session is using link-local addresses, but the ACL permits global addresses.
Why it's wrong here
Incorrect because the scenario states the peering uses global unicast addresses.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that ACLs for BGP peering should permit the neighbor's link-local address when using IPv6, but the trap here is that the BGP session is configured with global unicast addresses, so the ACL must permit the global unicast address, not the link-local address.
Trap categories for this question
Scenario analysis trap
Incorrect because the scenario states the peering uses global unicast addresses.
Detailed technical explanation
How to think about this question
BGP over IPv6 typically uses TCP port 179, and the source address of the TCP packets must match the neighbor statement's configured address. If an ACL permits only the link-local address but the BGP neighbor is defined with a global unicast address, the TCP SYN packets from the global address are dropped, preventing the BGP session from forming. This scenario often arises when engineers mistakenly apply ACLs based on the wrong address scope, forgetting that BGP uses the update-source interface's address (global or link-local) as specified in the neighbor command.
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.
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.
- →
IPv6 Traffic Filtering and uRPF — study guide chapter
Learn the concepts, then practise the questions
- →
IPv6 Traffic Filtering and uRPF practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
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.
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 ACL is blocking TCP packets from the neighbor's global unicast address because it only permits traffic from the link-local address. — The BGP session is flapping because the inbound ACL on R1 permits only TCP port 179 traffic sourced from the neighbor's link-local address, but the BGP peering is configured using global unicast addresses. Therefore, TCP packets from R2's global unicast address are dropped by the ACL, causing the BGP session to fail with a 'Notification sent: 3/2 (Update malformed)' error, as the TCP connection cannot be established or maintained.
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 →
Last reviewed: Jun 24, 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.
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.
Sign in to join the discussion.