hardMultiple ChoiceObjective-mapped
300-410 Practice Question: Is troubleshooting a BGP IPv6 peering issue…
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?
⚠ Common exam trap
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.
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.
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.
- ✗
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.
Why it's wrong here
Incorrect because ND messages are not directly related to BGP TCP connections; the issue is TCP port 179.
- ✗
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.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.