Question 1,725 of 2,152

IP SLA →hardMultiple ChoiceObjective-mapped

IP SLA Track Flapping Over GRE Tunnel: Root Cause

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.

An engineer configures IP SLA to track a route to a server that is reachable via a GRE tunnel. Router R1 has:

ip sla 9

icmp-echo 172.16.0.1 source-ip 10.0.0.1 frequency 10

ip sla schedule 9 life forever start-time now

track 9 ip sla 9 reachability

ip route 192.168.2.0 255.255.255.0 Tunnel0 track 9

The GRE tunnel is up, but the route is flapping. Show output on R1:

show interface Tunnel0 Tunnel0 is up, line protocol is up

show track 9

Track 9
  IP SLA 9 reachability

Reachability is Up 10 changes, last change 00:00:05

What is the root cause?

A
The GRE tunnel has a path MTU issue that causes ICMP packets to be fragmented or dropped, leading to intermittent probe failures.
GRE adds overhead, and if the path MTU is not adjusted, ICMP packets may be dropped due to fragmentation, causing the track to flap.
B
The IP SLA frequency is too high, causing the router to overload.
Why wrong: Frequency of 10 seconds is standard.
C
The source IP 10.0.0.1 is not reachable from the tunnel destination.
Why wrong: The tunnel is up, so reachability is not the issue.
D
The track 9 is not correctly associated with the route.
Why wrong: The association is correct; the flapping is due to the probe.

Full breakdown with real-world context →

Quick Answer

The root cause is a path MTU issue within the GRE tunnel causing ICMP echo probes to be fragmented or dropped, leading to intermittent IP SLA failures and subsequent track flapping. Even though the tunnel interface shows as up/up, the additional GRE encapsulation overhead reduces the effective MTU, so if the path between the tunnel endpoints does not support IP fragmentation or if ICMP unreachable messages are blocked, the IP SLA packets exceed the maximum segment size and are silently discarded. This creates a scenario where the track toggles between Up and Down every few seconds, as seen in the show track output with 10 changes in just five seconds. On the Cisco CCNP ENARSI 300-410 exam, this question tests your ability to differentiate between a tunnel interface state and the actual end-to-end reachability of the payload traffic—a common trap is assuming a “up/up” interface guarantees probe success. Remember the memory tip: “GRE adds 24 bytes; if your path doesn’t fragment, your SLA will fragment your track.”

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 GRE tunnel has a path MTU issue that causes ICMP packets to be fragmented or dropped, leading to intermittent probe failures.

The IP SLA probe is successful, but the track is flapping. This could be due to the IP SLA probe itself being intermittent, perhaps because the GRE tunnel is using a path that has high latency or packet loss. The correct answer is that the IP SLA probe is being affected by the GRE tunnel's path MTU or fragmentation issues, causing some probes to be dropped.

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 GRE tunnel has a path MTU issue that causes ICMP packets to be fragmented or dropped, leading to intermittent probe failures.
Why this is correct
GRE adds overhead, and if the path MTU is not adjusted, ICMP packets may be dropped due to fragmentation, causing the track to flap.
Related concept
Read the scenario before looking for a memorised answer.
✗
The IP SLA frequency is too high, causing the router to overload.
Why it's wrong here
Frequency of 10 seconds is standard.
✗
The source IP 10.0.0.1 is not reachable from the tunnel destination.
Why it's wrong here
The tunnel is up, so reachability is not the issue.
✗
The track 9 is not correctly associated with the route.
Why it's wrong here
The association is correct; the flapping is due to the probe.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.

Detailed technical explanation

How to think about this question

This question should be treated as a scenario, not a definition check. Identify the problem, the constraint and the best action. Then compare each option against those facts.

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.
Use explanations to understand the rule behind the answer.

TExam Day Tips

Underline the problem statement mentally.
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 practitioner preparing for the 300-410 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.

What to study next

Got this wrong? Here's your next step.

Identify which 300-410 exam domain this question belongs to, then review the specific concept being tested. Practise related questions in that domain and focus on understanding why each wrong answer is tempting — not just why the correct answer is right.

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FAQ

Questions learners often ask

What does this 300-410 question test?

IP SLA — This question tests IP SLA — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: The GRE tunnel has a path MTU issue that causes ICMP packets to be fragmented or dropped, leading to intermittent probe failures. — The IP SLA probe is successful, but the track is flapping. This could be due to the IP SLA probe itself being intermittent, perhaps because the GRE tunnel is using a path that has high latency or packet loss. The correct answer is that the IP SLA probe is being affected by the GRE tunnel's path MTU or fragmentation issues, causing some probes to be dropped.

What should I do if I get this 300-410 question wrong?

What is the key concept behind this question?

Read the scenario before looking for a memorised answer.

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