The answer is the inbound IPv6 ACL blocking non-BGP return traffic, not uRPF. When R1 sends traffic to 2001:db8:1::1, the destination sends back ICMP echo replies, but the ACL applied inbound on R1’s interface to R2 permits only TCP port 179, so these return packets are dropped. Strict mode uRPF checks the source address of incoming packets against the FIB, but here the return traffic’s source (2001:db8:1::1) is reachable via the BGP-learned route, so uRPF would pass it—the ACL is the sole culprit. On the CCNP ENARSI 300-410 exam, this tests your ability to distinguish ACL filtering from uRPF behavior, a common trap where candidates blame reverse path forwarding instead of the access list. Remember: an inbound ACL filters what enters the interface, so even legitimate return traffic from a BGP-learned destination is blocked unless explicitly permitted. Memory tip: “ACL first, uRPF second—if the ACL drops it, uRPF never sees it.”
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.
A dual-stack network uses BGP for IPv6 between two ISPs. R1 (AS 100) receives a full BGP table from R2 (AS 200). R1 has an IPv6 ACL applied inbound on the interface to R2 that permits only BGP (TCP 179) and denies all other traffic. R1 also has uRPF configured in strict mode on the same interface. R1's BGP table has a route to 2001:db8:1::/48 with next-hop 2001:db8:2::2. R1's routing table shows the route, but traffic from R1 to 2001:db8:1::1 fails. R1 shows 'show ipv6 cef 2001:db8:1::/48' points to 2001:db8:2::2 via the interface to R2. 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 ACL on R1 blocks the return traffic from the destination, which is not BGP, causing the ping to fail.
uRPF strict mode checks the source address of outgoing packets? No, it checks incoming packets. For traffic from R1 to the destination, the source is R1's own address. When the return traffic comes back, uRPF checks the source of the return packet. But the issue is that the ACL blocks all traffic except BGP. When R1 sends data to the destination, the return traffic is blocked by the ACL because it is not BGP. The uRPF is not the issue; the ACL is blocking the return traffic.
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 ACL on R1 blocks the return traffic from the destination, which is not BGP, causing the ping to fail.
Why this is correct
The ACL permits only BGP. Return traffic (ICMPv6 echo reply) is blocked, so the ping fails.
Related concept
Static NAT maps one inside address to one outside address.
✗
uRPF strict mode drops the outgoing traffic because the source address is not reachable via the interface.
Why it's wrong here
uRPF checks incoming traffic, not outgoing.
✗
The next-hop 2001:db8:2::2 is not reachable due to a missing ND entry.
Why it's wrong here
If ND were missing, the route would not be installed in CEF.
✗
BGP next-hop resolution fails because the next-hop is not in the FIB.
Why it's wrong here
The route is installed, so next-hop resolution succeeded.
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 ACL on R1 blocks the return traffic from the destination, which is not BGP, causing the ping to fail. — uRPF strict mode checks the source address of outgoing packets? No, it checks incoming packets. For traffic from R1 to the destination, the source is R1's own address. When the return traffic comes back, uRPF checks the source of the return packet. But the issue is that the ACL blocks all traffic except BGP. When R1 sends data to the destination, the return traffic is blocked by the ACL because it is not BGP. The uRPF is not the issue; the ACL is blocking the return traffic.
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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