- A
The route-map set distance 100 overrides the default AD, making the redistributed route preferred over the internal route (AD 90).
Why wrong: 100 is higher than 90, so internal route is still preferred.
- B
The internal EIGRP route is not present in R3's routing table due to a distribute-list inbound from R1, so the redistributed route with AD 100 is the only path.
If the internal route is filtered, the redistributed route becomes the best path, causing suboptimal routing.
- C
The redistributed route has AD 170 by default, and the route-map is ignored because redistribution from BGP always uses AD 170.
Why wrong: Route-map can change AD.
- D
R3 prefers routes with lower metric, not AD, and the redistributed route has a better metric.
Why wrong: AD is checked first; if AD is equal, metric is compared.
Quick Answer
The answer is that a distribute-list filtering the internal EIGRP route on R3 is the most likely cause of suboptimal routing. EIGRP internal routes have an administrative distance of 90, while external routes redistributed into EIGRP default to AD 170. When a route-map sets the AD to 100, that redistributed route is still less preferred than the internal route because 100 is higher than 90, so the internal route should win. This question tests your understanding of how administrative distance interacts with route redistribution and filtering in the Cisco CCNP ENARSI 300-410 exam. The common trap is assuming that setting AD to 100 automatically makes the redistributed route preferred, when in fact the internal route (AD 90) remains superior—unless it is missing entirely due to an inbound distribute-list on R3. A key memory tip: “90 internal, 170 external; a set distance above 90 still loses to the internal.”
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. 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.
Router R1 and R2 are running EIGRP as the IGP, and R1 is redistributing a connected subnet 10.1.1.0/24 into EIGRP. R2 also runs BGP with an external peer, and BGP is redistributing the same prefix 10.1.1.0/24 into EIGRP with a route-map that sets the administrative distance to 100. On R3, a downstream EIGRP router, 'show ip route 10.1.1.0' shows the route via R2. What is the most likely cause of suboptimal routing?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue:
"most likely"Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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 internal EIGRP route is not present in R3's routing table due to a distribute-list inbound from R1, so the redistributed route with AD 100 is the only path.
EIGRP internal routes have AD 90, external routes AD 170. Redistribution from BGP into EIGRP creates external routes (AD 170) unless a route-map changes the distance. Setting AD to 100 makes the redistributed route preferred over the original internal route (AD 90) because 100 > 90, so the internal route should be preferred. However, if the internal route is not present due to a filter, or if the distance is set lower than 90, the redistributed route is chosen. The correct answer is that the route-map set distance 100 is higher than 90, so the internal route (AD 90) is still preferred; the issue is that the internal route is being suppressed by a distribute-list on R3.
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 route-map set distance 100 overrides the default AD, making the redistributed route preferred over the internal route (AD 90).
Why it's wrong here
100 is higher than 90, so internal route is still preferred.
- ✓
The internal EIGRP route is not present in R3's routing table due to a distribute-list inbound from R1, so the redistributed route with AD 100 is the only path.
Why this is correct
If the internal route is filtered, the redistributed route becomes the best path, causing suboptimal routing.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The redistributed route has AD 170 by default, and the route-map is ignored because redistribution from BGP always uses AD 170.
Why it's wrong here
Route-map can change AD.
- ✗
R3 prefers routes with lower metric, not AD, and the redistributed route has a better metric.
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 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.
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?
Administrative Distance — This question tests Administrative Distance — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The internal EIGRP route is not present in R3's routing table due to a distribute-list inbound from R1, so the redistributed route with AD 100 is the only path. — EIGRP internal routes have AD 90, external routes AD 170. Redistribution from BGP into EIGRP creates external routes (AD 170) unless a route-map changes the distance. Setting AD to 100 makes the redistributed route preferred over the original internal route (AD 90) because 100 > 90, so the internal route should be preferred. However, if the internal route is not present due to a filter, or if the distance is set lower than 90, the redistributed route is chosen. The correct answer is that the route-map set distance 100 is higher than 90, so the internal route (AD 90) is still preferred; the issue is that the internal route is being suppressed by a distribute-list on R3.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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