- A
The IP SLA probe uses a source IP that is not reachable from the server, causing the probe to fail and the tracked route to be removed unnecessarily.
The source IP 192.168.1.1 may not be in the routing table of the server or intermediate routers, so echo replies never return, causing the track to go down.
- B
The frequency of the IP SLA is too high, causing the router to overload and drop the tracked route.
Why wrong: A frequency of 10 seconds is standard and should not cause overload.
- C
The delay down 5 up 5 is too short, causing flapping of the tracked route.
Why wrong: The delay values are reasonable and would not cause intermittent reachability by themselves.
- D
The server 10.10.10.10 is not responding to ICMP echo requests due to a firewall.
Why wrong: While possible, the question indicates the server is critical and should respond; the root cause is more likely a routing issue with the source IP.
Quick Answer
The answer is that the IP SLA source IP routability is causing a false negative, because the probe sourced from 192.168.1.1 cannot reach the server 10.10.10.10, even though the next-hop 192.168.1.2 is perfectly reachable. This happens when the server’s return traffic has no route back to the probe’s source IP, so the ICMP echo reply never arrives, the probe fails, and the tracked default route is incorrectly removed. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IP SLA reachability tracks end-to-end path health, not just next-hop availability—a common trap is assuming a failed probe always means the gateway is down. The key is to verify that the source IP used in the ip sla icmp-echo command is routable from the target’s perspective, especially in networks with asymmetric routing or route summarization. Memory tip: “Source IP must be reachable, not just the gateway.”
300-410 IP SLA Practice Question
This 300-410 practice question tests your understanding of ip sla. 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 large enterprise network is experiencing intermittent reachability to a critical server (10.10.10.10). Router R1 has the following relevant configuration:
ip sla 1
icmp-echo 10.10.10.10 source-ip 192.168.1.1 frequency 10
ip sla schedule 1 life forever start-time now track 1 ip sla 1 reachability
delay down 5 up 5
ip route 0.0.0.0 0.0.0.0 192.168.1.2 track 1 Router R2 (the next-hop) shows:
show ip route | include 10.10.10.10
S 10.10.10.0/24 [1/0] via 10.0.0.1
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 IP SLA probe uses a source IP that is not reachable from the server, causing the probe to fail and the tracked route to be removed unnecessarily.
The IP SLA probes are sourced from 192.168.1.1, but the server 10.10.10.10 is in a different subnet. The return path from the server might not reach 192.168.1.1 due to route summarization or asymmetric routing. However, the key issue is that the tracked default route is removed when the probe fails, but the probe failure could be due to the server's response not being routed back correctly, not because the next-hop is unreachable. The correct answer is that the source IP of the probe is not routable from the server's perspective, causing false negatives.
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 IP SLA probe uses a source IP that is not reachable from the server, causing the probe to fail and the tracked route to be removed unnecessarily.
Why this is correct
The source IP 192.168.1.1 may not be in the routing table of the server or intermediate routers, so echo replies never return, causing the track to go down.
Related concept
CIDR notation defines the prefix length.
- ✗
The frequency of the IP SLA is too high, causing the router to overload and drop the tracked route.
Why it's wrong here
A frequency of 10 seconds is standard and should not cause overload.
- ✗
The delay down 5 up 5 is too short, causing flapping of the tracked route.
Why it's wrong here
The delay values are reasonable and would not cause intermittent reachability by themselves.
- ✗
The server 10.10.10.10 is not responding to ICMP echo requests due to a firewall.
Why it's wrong here
While possible, the question indicates the server is critical and should respond; the root cause is more likely a routing issue with the source IP.
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?
IP SLA — This question tests IP SLA — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The IP SLA probe uses a source IP that is not reachable from the server, causing the probe to fail and the tracked route to be removed unnecessarily. — The IP SLA probes are sourced from 192.168.1.1, but the server 10.10.10.10 is in a different subnet. The return path from the server might not reach 192.168.1.1 due to route summarization or asymmetric routing. However, the key issue is that the tracked default route is removed when the probe fails, but the probe failure could be due to the server's response not being routed back correctly, not because the next-hop is unreachable. The correct answer is that the source IP of the probe is not routable from the server's perspective, causing false negatives.
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 19, 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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