mediumMultiple ChoiceObjective-mapped
300-410 Practice Question: Runs the following command on Router R1: R1# show…
A network engineer runs the following command on Router R1:
R1# show ipv6 mtu
IPv6 MTU per interface:
Interface MTU
Tunnel0 1476 Tunnel1 1476 GigabitEthernet0/0 1500
Based on this output, what is the most likely reason for the reduced MTU on the tunnel interfaces?
⚠ Common exam trap
Cisco often tests the exact overhead values for different tunnel types (GRE vs. IPsec vs. 6to4) to see if candidates know that GRE adds exactly 24 bytes, while IPsec adds more and 6to4 adds only 20 bytes.
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 tunnels are using GRE encapsulation, which adds 24 bytes of overhead.
The output shows an MTU of 1476 on the tunnel interfaces, which is exactly 24 bytes less than the 1500-byte MTU of the physical GigabitEthernet0/0 interface. GRE encapsulation adds a 4-byte outer IP header and a 4-byte GRE header (total 24 bytes with the inner IP header), reducing the payload MTU. This is the default behavior when a tunnel is configured without a manual MTU override, making option B correct.
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 tunnels are configured with a manual MTU.
Why it's wrong here
While possible, the default behavior is to reduce MTU for encapsulation.
- ✓
The tunnels are using GRE encapsulation, which adds 24 bytes of overhead.
Why this is correct
GRE adds 4 bytes to the 20-byte IPv4 header, totaling 24 bytes overhead.
- ✗
The tunnels are using IPsec, which adds more overhead.
Why it's wrong here
IPsec overhead is typically larger and variable; 24 bytes is consistent with GRE.
- ✗
The tunnels are 6to4 tunnels, which do not reduce MTU.
Why it's wrong here
6to4 tunnels also reduce MTU due to IPv4 encapsulation.
Go deeper
Related to this question
About these practice questions
This 300-410 question is part of Courseiva's 1,966-question bank — original exam-style content with full explanations and wrong-answer analysis, never real exam questions or exam dumps. Learn why practice questions differ from exam dumps →
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.