- A
The summary route 2001:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface.
uRPF strict mode requires the source address to be reachable via the same interface. The summary route points to R1, but the source is directly connected, causing a mismatch.
- B
R2's uRPF is configured in loose mode, which requires a matching route in the FIB, but the summary route is not installed.
Why wrong: Loose mode only requires a route in the FIB, not necessarily via the same interface. The issue is strict mode.
- C
EIGRP redistribution of the summary route creates a routing loop, causing uRPF to fail.
Why wrong: There is no loop; the summary is via Null0 on R1, not a loop.
- D
The host behind R2 has an incorrect source address, causing uRPF to drop all traffic.
Why wrong: The source address is correct for the subnet, but uRPF checks routing, not host configuration.
EIGRP Summary Route Causes uRPF Strict Mode Drop
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 summarization. Router R1 has a summary route 2001:db8:1::/48 via Null0 redistributed into EIGRP. Router R2 receives this summary and has a more specific route 2001:db8:1:1::/64 learned via a different interface. R2's IPv6 uRPF is configured in strict mode on the interface facing R1. Traffic from a host behind R2 destined to 2001:db8:1:2::1 is being dropped. R2 shows 'ipv6 cef' indicates the summary route points to R1, but uRPF checks fail. What is the root cause?
Quick Answer
The answer is that uRPF strict mode drops the traffic because the summary route 2001:db8:1::/48 on R1 creates a less specific path pointing back to R1, making the source address appear unreachable via the incoming interface. When R2 receives a packet sourced from its own directly connected subnet, uRPF strict mode performs a reverse path lookup: it checks whether the source address is reachable via the same interface the packet arrived on. Because R2’s routing table shows the summary route pointing to R1 for the entire /48 prefix, the source address 2001:db8:1:2::1 is matched by that less specific route, not the more specific /64, so uRPF sees the return path going out a different interface and drops the packet. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how route summarization interacts with unicast reverse path forwarding—a common trap is forgetting that uRPF uses the best matching route, not the most specific, for its source reachability check. Remember the mnemonic: “Summary steals the source check.”
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:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface.
The correct answer is A because R2's strict mode uRPF checks that the source address of incoming traffic is reachable via the exact interface on which the packet arrived. When R2 receives a packet from a host behind it destined to 2001:db8:1:2::1, the source address is from the host's subnet (e.g., 2001:db8:1:2::/64). R2's FIB has a less specific summary route 2001:db8:1::/48 pointing to R1 (via the interface facing R1), but the more specific /64 route is learned via a different interface. Strict uRPF requires a matching route in the FIB that points back out the same interface the packet arrived on; since the summary route points to R1's interface, uRPF fails, dropping the traffic.
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:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface.
Why this is correct
uRPF strict mode requires the source address to be reachable via the same interface. The summary route points to R1, but the source is directly connected, causing a mismatch.
Related concept
Read the scenario before looking for a memorised answer.
- ✗
R2's uRPF is configured in loose mode, which requires a matching route in the FIB, but the summary route is not installed.
Why it's wrong here
Loose mode only requires a route in the FIB, not necessarily via the same interface. The issue is strict mode.
- ✗
EIGRP redistribution of the summary route creates a routing loop, causing uRPF to fail.
Why it's wrong here
There is no loop; the summary is via Null0 on R1, not a loop.
- ✗
The host behind R2 has an incorrect source address, causing uRPF to drop all traffic.
Why it's wrong here
The source address is correct for the subnet, but uRPF checks routing, not host configuration.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between strict and loose uRPF modes, and the trap here is that candidates overlook how a less specific summary route can satisfy the FIB lookup but still fail strict mode's interface check, leading them to incorrectly blame routing loops or source address issues.
Detailed technical explanation
How to think about this question
Strict uRPF (RFC 3704) performs a reverse path lookup: for each incoming packet, it checks the FIB for the source address and requires the best matching route to point out the same interface the packet arrived on. When a summary route (e.g., /48) covers the source but points to a different interface than the more specific route (e.g., /64), the lookup returns the summary route, causing a mismatch. This is a common issue in networks with route summarization and asymmetric routing, where uRPF strict mode can drop legitimate 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 segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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:db8:1::/48 on R1 causes R2 to have a less specific route pointing to R1, making uRPF think the source address is not reachable via the incoming interface. — The correct answer is A because R2's strict mode uRPF checks that the source address of incoming traffic is reachable via the exact interface on which the packet arrived. When R2 receives a packet from a host behind it destined to 2001:db8:1:2::1, the source address is from the host's subnet (e.g., 2001:db8:1:2::/64). R2's FIB has a less specific summary route 2001:db8:1::/48 pointing to R1 (via the interface facing R1), but the more specific /64 route is learned via a different interface. Strict uRPF requires a matching route in the FIB that points back out the same interface the packet arrived on; since the summary route points to R1's interface, uRPF fails, dropping the traffic.
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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