- 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.
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.”
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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?
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.
uRPF strict mode checks that the source address of incoming packets is reachable via the same interface. For traffic sourced from R2's subnet (e.g., 2001:db8:1:1::/64), the return route via the summary points to R1, but the source is directly connected. This asymmetry causes uRPF to drop the packet because the source is not reachable via the incoming interface.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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
CIDR notation defines the prefix length.
- ✗
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: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
- CIDR notation defines the prefix length.
- Block size helps identify subnet boundaries.
- Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
- The required host count determines the smallest suitable subnet.
TExam Day Tips
- Write the block size before choosing the subnet.
- Check whether the question asks for hosts, subnets or a specific address range.
- Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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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 — CIDR notation defines the prefix length..
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. — uRPF strict mode checks that the source address of incoming packets is reachable via the same interface. For traffic sourced from R2's subnet (e.g., 2001:db8:1:1::/64), the return route via the summary points to R1, but the source is directly connected. This asymmetry causes uRPF to drop the packet because the source is not reachable via the incoming interface.
What should I do if I get this 300-410 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.
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