- A
The ACL on the VRF A interface blocks data traffic from VRF B, which is forwarded via the leaked route.
Traffic from VRF B is forwarded into VRF A and hits the inbound ACL, which permits only OSPFv3.
- B
Route leaking is not configured correctly; the route-target import/export is missing.
Why wrong: If leaking were broken, the ping from VRF A would also fail.
- C
The static default route in VRF B has an incorrect next-hop address.
Why wrong: If the next-hop were wrong, the ping from VRF A would also fail.
- D
uRPF is enabled on the VRF A interface and drops traffic from VRF B due to source address mismatch.
Why wrong: uRPF is not mentioned in the configuration; the ACL is the issue.
Question 791 of 2,152
IPv6 ACL in VRF Blocking Inter-VRF Traffic: Route Leaking and ACL on Source VRF
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.
An enterprise uses VRF-lite with IPv6. VRF A on R1 leaks routes to VRF B using route-target import/export. R1 has an IPv6 ACL applied inbound on the interface in VRF A that permits only OSPFv3 and denies all other traffic. R1's VRF B has a static default route pointing to a next-hop in VRF A. Traffic from VRF B to the internet fails. R1 shows 'ping vrf B 2001:db8:2::1' fails, but 'ping vrf A 2001:db8:2::1' succeeds. What is the root cause?
Quick Answer
The answer is that the IPv6 ACL applied inbound on the VRF A interface is blocking the inter-VRF traffic from VRF B because the leaked route causes packets to enter VRF A, where they are subject to the ACL’s deny-all-except-OSPFv3 rule. When VRF B uses its static default route pointing to a next-hop in VRF A, the data traffic crosses into VRF A and is evaluated by the inbound ACL, which drops non-OSPFv3 packets like ICMPv6 echo requests. This scenario tests your understanding of how route leaking interacts with interface ACLs in VRF-lite environments—a common trap on the Cisco CCNP ENARSI 300-410 exam is assuming ACLs only filter traffic originating from within the VRF, when in fact they filter all inbound traffic on the interface, including traffic arriving via leaked routes. Remember: an ACL on the source VRF’s interface does not distinguish between locally-sourced traffic and traffic that enters via a leaked route. Memory tip: “Leak and block—if the ACL’s on the door, it checks every knock, not just the neighbor’s.”
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 on the VRF A interface blocks data traffic from VRF B, which is forwarded via the leaked route.
The ACL on the VRF A interface is configured to permit only OSPFv3 (IPv6 routing protocol traffic) and deny all other traffic. When VRF B leaks routes to VRF A, data traffic from VRF B destined to the internet is forwarded via the leaked route and enters the VRF A interface. This data traffic is not OSPFv3, so it is denied by the ACL, causing the ping to fail. The ping from VRF A succeeds because it originates within VRF A and does not traverse the ACL inbound on the same interface.
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 on the VRF A interface blocks data traffic from VRF B, which is forwarded via the leaked route.
Why this is correct
Traffic from VRF B is forwarded into VRF A and hits the inbound ACL, which permits only OSPFv3.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
Route leaking is not configured correctly; the route-target import/export is missing.
Why it's wrong here
If leaking were broken, the ping from VRF A would also fail.
- ✗
The static default route in VRF B has an incorrect next-hop address.
Why it's wrong here
If the next-hop were wrong, the ping from VRF A would also fail.
- ✗
uRPF is enabled on the VRF A interface and drops traffic from VRF B due to source address mismatch.
Why it's wrong here
uRPF is not mentioned in the configuration; the ACL is the issue.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that route leaking bypasses interface ACLs, when in fact ACLs are applied to the data plane traffic that uses the leaked routes, not just to the routing protocol updates.
Detailed technical explanation
How to think about this question
In VRF-lite with route leaking, traffic between VRFs is forwarded based on leaked routes, but the actual data packets must still pass through any inbound ACLs on the interface they enter. OSPFv3 uses protocol number 89 for IPv6, so an ACL permitting only OSPFv3 will block all other IP traffic, including ICMPv6 echo requests used by ping. This scenario is common when an ACL is mistakenly applied to filter control plane traffic but inadvertently blocks data plane traffic.
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
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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: The ACL on the VRF A interface blocks data traffic from VRF B, which is forwarded via the leaked route. — The ACL on the VRF A interface is configured to permit only OSPFv3 (IPv6 routing protocol traffic) and deny all other traffic. When VRF B leaks routes to VRF A, data traffic from VRF B destined to the internet is forwarded via the leaked route and enters the VRF A interface. This data traffic is not OSPFv3, so it is denied by the ACL, causing the ping to fail. The ping from VRF A succeeds because it originates within VRF A and does not traverse the ACL inbound on the same interface.
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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