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.”
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.
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
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: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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
Static NAT maps one inside address to one outside address.
✗
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: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
IP SLA — This question tests IP SLA — Static NAT maps one inside address to one outside address..
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?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 300-410 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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