- A
The summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1.
R2 forwards traffic to R1 based on the summary route, but R1's static route to Null0 drops it. The ACL is irrelevant to this failure.
- B
The ACL on R2 blocks the return traffic from R1, causing asymmetric routing.
Why wrong: The ACL is inbound on R2, so it filters traffic coming into R2, not going out.
- C
uRPF loose mode drops the packet because the source address is not in the FIB.
Why wrong: Loose mode only requires a route in the FIB, which exists.
- D
EIGRP redistribution of the static route creates a routing loop between R1 and R2.
Why wrong: There is no loop; the traffic is simply dropped at R1.
Question 2,120 of 2,152
ACL Permitting Only EIGRP Blocks Data Traffic After Redistribution
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 EIGRP for IPv6 with route redistribution from a static route. R1 has a static route 2001:db8:0::/32 via Null0 redistributed into EIGRP. R2 receives this route and has a more specific route 2001:db8:1::/32 via a different interface. R2 has an IPv6 ACL applied inbound on the interface facing R1 that permits only EIGRP and denies all other traffic. R2's uRPF is configured in loose mode. Traffic from R2 to 2001:db8:2::1 fails. R2 shows 'show ipv6 route' has both routes, but 'show ipv6 cef' shows the summary route for 2001:db8:2::1 pointing to R1. What is the root cause?
Quick Answer
The root cause is that the summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1. This occurs because R2’s CEF table shows the less specific summary route for the destination 2001:db8:2::1, forwarding traffic out the interface toward R1, where the Null0 static route silently discards it. The IPv6 ACL permitting only EIGRP is a red herring—since it is applied inbound on R2’s interface facing R1, it does not filter outbound data traffic from R2, and uRPF loose mode only verifies a return route exists, not the forwarding interface. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to distinguish between ACL filtering direction and routing blackholing, a common trap where candidates blame the ACL instead of the summary route. Remember: when EIGRP redistribution introduces a Null0 summary, it can override more specific paths in the FIB, silently dropping traffic regardless of ACLs or uRPF.
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 summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1.
The correct answer is A because R1's static route 2001:db8:0::/32 via Null0 is redistributed into EIGRP, and R2 learns this summary route. When R2 sends traffic to 2001:db8:2::1, the CEF table (show ipv6 cef) shows the longest prefix match for 2001:db8:2::1 is the summary 2001:db8:0::/32 pointing to R1, not the more specific 2001:db8:1::/32. R1 then forwards the packet to Null0 (a virtual interface that drops traffic), causing the failure. The more specific route 2001:db8:1::/32 on R2 is not used because 2001:db8:2::1 does not fall within that prefix.
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 summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1.
Why this is correct
R2 forwards traffic to R1 based on the summary route, but R1's static route to Null0 drops it. The ACL is irrelevant to this failure.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
The ACL on R2 blocks the return traffic from R1, causing asymmetric routing.
Why it's wrong here
The ACL is inbound on R2, so it filters traffic coming into R2, not going out.
- ✗
uRPF loose mode drops the packet because the source address is not in the FIB.
Why it's wrong here
Loose mode only requires a route in the FIB, which exists.
- ✗
EIGRP redistribution of the static route creates a routing loop between R1 and R2.
Why it's wrong here
There is no loop; the traffic is simply dropped at R1.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between the routing table (RIB) and CEF (FIB), where candidates assume that a more specific route in the RIB will be used for forwarding, but CEF may use a different prefix if the destination does not match the more specific prefix exactly.
Detailed technical explanation
How to think about this question
CEF (Cisco Express Forwarding) uses the FIB (Forwarding Information Base) for packet switching, and the FIB performs longest prefix match independently of the routing table. In this scenario, the more specific route 2001:db8:1::/32 is in the routing table but does not cover 2001:db8:2::1, so CEF correctly uses the summary route 2001:db8:0::/32. The Null0 interface is a virtual interface that drops all traffic sent to it, commonly used to prevent routing loops when summarizing or discarding traffic. Real-world scenarios often involve misconfigured summarization or redistribution of Null0 routes causing blackholing of traffic that should use a more specific path.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
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 summary route 2001:db8:0::/32 on R1 points to Null0, causing traffic to be dropped at R1. — The correct answer is A because R1's static route 2001:db8:0::/32 via Null0 is redistributed into EIGRP, and R2 learns this summary route. When R2 sends traffic to 2001:db8:2::1, the CEF table (show ipv6 cef) shows the longest prefix match for 2001:db8:2::1 is the summary 2001:db8:0::/32 pointing to R1, not the more specific 2001:db8:1::/32. R1 then forwards the packet to Null0 (a virtual interface that drops traffic), causing the failure. The more specific route 2001:db8:1::/32 on R2 is not used because 2001:db8:2::1 does not fall within that prefix.
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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