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IP SLA →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is a source IP mismatch in the IP SLA configuration. The probe is sourced from 10.0.0.1, but the DMVPN tunnel source is the physical interface IP, so the ICMP echo is sent out the physical interface rather than through the tunnel, failing to reach the destination tunnel IP of 172.16.0.1 and causing the track to go down. This scenario tests your understanding of how IP SLA probes interact with DMVPN overlay routing on the Cisco CCNP ENARSI 300-410 exam, where a common trap is assuming the tunnel is used simply because it is up. The key insight is that the source-ip must match the tunnel source interface IP for the probe to traverse the mGRE tunnel; otherwise, the router uses the routing table to forward the probe out the physical interface, which may lack a route to the spoke’s tunnel IP. Memory tip: “Source equals tunnel source” — always verify that the IP SLA source-ip matches the tunnel source address, not the tunnel IP itself.

300-410 IP SLA Practice Question

This 300-410 practice question tests your understanding of ip sla. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 company uses IP SLA to track a route to a remote network via a DMVPN tunnel. Router R1 (hub) has:

ip sla 5

icmp-echo 172.16.0.1 source-ip 10.0.0.1 frequency 10

ip sla schedule 5 life forever start-time now

track 5 ip sla 5 reachability

ip route 192.168.1.0 255.255.255.0 Tunnel0 track 5

Tunnel0 is a DMVPN tunnel to spoke R2. The tunnel is up, but the route is intermittently removed. Show output on R1:

show ip route 192.168.1.0 % Subnet not in table

show track 5

Track 5
  IP SLA 5 reachability

Reachability is Down 2 changes, last change 00:00:05

What is the root cause?

Question 1hardmultiple choice
Read the full VPN explanation →

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 source IP of the IP SLA probe is not the tunnel source, so the probe is sent out the physical interface and does not reach the destination.

The IP SLA probe is sent to 172.16.0.1, which is the tunnel IP of R2. However, the DMVPN tunnel might be using a different encapsulation (e.g., mGRE) and the probe might be sent over the physical interface instead of the tunnel, or the tunnel might be in a VRF. The correct answer is that the IP SLA probe is sourced from 10.0.0.1, which is not the tunnel source, so the probe goes out the physical interface and may not reach 172.16.0.1, causing the track to go down.

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 source IP of the IP SLA probe is not the tunnel source, so the probe is sent out the physical interface and does not reach the destination.

    Why this is correct

    The probe should be sourced from the tunnel source IP to ensure it goes through the tunnel; otherwise, it may be dropped or not reach the target.

    Related concept

    CIDR notation defines the prefix length.

  • ✗

    The DMVPN tunnel is down because the spoke is not reachable.

    Why it's wrong here

    The tunnel is up per the scenario, but the probe fails due to source IP mismatch.

  • ✗

    The IP SLA frequency is too low, causing the track to go down after 10 seconds of no response.

    Why it's wrong here

    Frequency is not the issue; the probe is not reaching the target at all.

  • ✗

    The route is removed because the track is down, but the track is down due to a routing loop in the DMVPN network.

    Why it's wrong here

    There is no evidence of a routing loop; the issue is with the probe source.

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.

Trap categories for this question

  • Scenario analysis trap

    The tunnel is up per the scenario, but the probe fails due to source IP mismatch.

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 source IP of the IP SLA probe is not the tunnel source, so the probe is sent out the physical interface and does not reach the destination. — The IP SLA probe is sent to 172.16.0.1, which is the tunnel IP of R2. However, the DMVPN tunnel might be using a different encapsulation (e.g., mGRE) and the probe might be sent over the physical interface instead of the tunnel, or the tunnel might be in a VRF. The correct answer is that the IP SLA probe is sourced from 10.0.0.1, which is not the tunnel source, so the probe goes out the physical interface and may not reach 172.16.0.1, causing the track to go down.

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

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