hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer is troubleshooting an IPv6 manual…
An engineer is troubleshooting an IPv6 manual tunnel between two routers that are not directly connected (the tunnel endpoints are separated by an IPv4 network). The tunnel is up, but the engineer notices that the tunnel interface flaps every few minutes. The show interfaces tunnel output shows input errors and CRC errors. What is the most likely cause?
⚠ Common exam trap
Cisco often tests the concept that tunnel flapping with CRC errors points to MTU/fragmentation issues, not reachability or encapsulation mismatches, which would prevent the tunnel from coming up at all.
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 IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
The tunnel interface shows input errors and CRC errors, which are classic signs of fragmentation and reassembly issues. When the IPv4 path between tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (the IPv4 header), packets exceeding the path MTU are fragmented. Fragmentation increases the risk of packet loss and CRC errors, causing the tunnel to flap as the interface resets or routing reconverges.
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 IPv4 path between the tunnel endpoints has a lower MTU than the tunnel interface MTU plus 20 bytes (IP header), causing fragmentation and packet loss.
Why this is correct
Correct because the tunnel MTU is typically 1500, but the IPv4 path may have an MTU of 1500 or less, leading to fragmentation when the 20-byte IP header is added. This causes errors and flapping.
- ✗
The tunnel keepalive is set too low, causing the tunnel to go down when a keepalive is lost.
Why it's wrong here
Incorrect because keepalive loss would cause the tunnel to go down, not just flap with CRC errors.
- ✗
The tunnel source interface is a loopback that is not reachable from the remote router.
Why it's wrong here
Incorrect because the tunnel is up, so the source is reachable.
- ✗
The tunnel is using GRE encapsulation, but the remote router is configured for ipv6ip.
Why it's wrong here
Incorrect because a mode mismatch would prevent the tunnel from coming up at all.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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