hardMultiple ChoiceObjective-mapped
300-410 Practice Question: A dual-stack network uses BGP for IPv6 between AS…
A dual-stack network uses BGP for IPv6 between AS 100 and AS 200. Router R1 (AS 100) has an inbound route-map that sets local preference to 200 for routes from R2 (AS 200). R1 also has an IPv6 ACL applied inbound that permits only BGP (TCP 179) and denies ICMPv6. R2 advertises a prefix 2001:db8:1::/48. R1's BGP table shows the prefix with local preference 200, but 'show ipv6 route' does not install it. R1 has uRPF strict mode on the interface facing R2. What is the root cause?
⚠ Common exam trap
Cisco often tests the dependency of IPv6 routing on ICMPv6 for neighbor discovery, leading candidates to overlook that denying ICMPv6 breaks next-hop resolution even when BGP sessions are up.
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 blocks ICMPv6, which is required for IPv6 neighbor discovery. Without ND, the next-hop address of the BGP route is unresolved, preventing route installation.
The ACL applied inbound on R1 permits only BGP (TCP port 179) and denies all other traffic, including ICMPv6. IPv6 neighbor discovery (ND) relies on ICMPv6 messages such as Neighbor Solicitation (NS) and Neighbor Advertisement (NA) to resolve the link-layer address of the next-hop router. Without ICMPv6, R1 cannot perform ND for the BGP next-hop address (R2's interface), so the next-hop remains unresolved. BGP routes with an unresolved next-hop are not installed into the IPv6 routing table, even though they appear in the BGP table with the correct local preference.
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 blocks ICMPv6, which is required for IPv6 neighbor discovery. Without ND, the next-hop address of the BGP route is unresolved, preventing route installation.
Why this is correct
BGP uses TCP, but the next-hop must be reachable via ND. Blocking ICMPv6 prevents ND, so the route is not installed.
- ✗
uRPF strict mode drops the BGP updates because the source address is not in the FIB.
Why it's wrong here
uRPF checks source of data packets, not BGP updates. BGP uses TCP, and uRPF is applied to transit traffic.
- ✗
The route-map sets local preference incorrectly, causing the route to be suppressed.
Why it's wrong here
Local preference 200 is valid and should prefer the route.
- ✗
R1's BGP process has 'soft-reconfiguration inbound' enabled, causing the route to be hidden.
Why it's wrong here
Soft-reconfiguration does not prevent route installation; it stores updates.
Visual reference
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Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
IPv6 Next Hop Resolution
IPv6 Next Hop Resolution is the process a router uses to determine the next router or destination device to which a packet should be forwarded, based on the destination IPv6 address and the routing table.
Key term
uRPF
Unicast Reverse Path Forwarding is a network security feature that verifies the source address of incoming packets to prevent IP spoofing attacks.
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JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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