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.
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.
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?
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 ACL on R2 blocks the data traffic from R2 to the destination because the traffic is sent out the interface facing R1 (due to the summary route), but the ACL is inbound on that interface. However, uRPF loose mode only checks if a route exists in the FIB, not the interface. The issue is that the ACL is blocking the outbound traffic? Actually, the ACL is inbound, so it filters incoming traffic. The traffic from R2 to the destination is outbound, so the ACL does not affect it. The real issue is that the summary route points to R1, but the destination is not reachable via R1 because R1's Null0 route drops it. The ACL is a red herring; the root cause is the summary route causing blackholing.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
Real-world example
How this comes up in practice
A small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Static NAT maps one inside address to one outside address..
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 ACL on R2 blocks the data traffic from R2 to the destination because the traffic is sent out the interface facing R1 (due to the summary route), but the ACL is inbound on that interface. However, uRPF loose mode only checks if a route exists in the FIB, not the interface. The issue is that the ACL is blocking the outbound traffic? Actually, the ACL is inbound, so it filters incoming traffic. The traffic from R2 to the destination is outbound, so the ACL does not affect it. The real issue is that the summary route points to R1, but the destination is not reachable via R1 because R1's Null0 route drops it. The ACL is a red herring; the root cause is the summary route causing blackholing.
What should I do if I get this 300-410 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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