The correct conclusion is that the MTU of 1400 bytes indicates this is a GRE or IPsec tunnel. This is because the default Ethernet MTU is 1500 bytes, and the reduction to 1400 bytes accounts for the encapsulation overhead added by GRE (typically 24 bytes) and IPsec (which can add 50–60+ bytes depending on the mode and encryption). On the ENCOR 350-401 exam, the show ip interface tunnel command is a key tool for verifying tunnel parameters, and a non-standard MTU like 1400 is a direct clue that encapsulation is in use—a common trap is assuming a tunnel is IPsec just because the MTU is lowered, but GRE alone also reduces it. To remember this, think of the “GRE/IPsec tunnel MTU show command” as your go-to diagnostic: if you see an MTU between 1376 and 1400, expect a tunnel overhead of 100–124 bytes, and always check the tunnel mode to distinguish between GRE and IPsec. A useful memory tip is “1400 for overhead, 1500 for normal”—any drop from 1500 signals encapsulation.
CCNP VPN Technologies Practice Question
This 350-401 practice question tests your understanding of vpn technologies. 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 network engineer runs the following command on Router R9:
R9# show ip interface tunnel 0
Tunnel0 is up, line protocol is up
Internet address is 10.0.0.9/24
Broadcast address is 255.255.255.255
Address determined by non-volatile memory
MTU is 1400 bytes
Helper address is not set
Directed broadcast forwarding is disabled
Outgoing access list is not set
Inbound access list is not set
Proxy ARP is disabled
Local Proxy ARP is disabled
Security level is default
Split horizon is enabled
ICMP redirects are always sent
ICMP unreachables are always sent
ICMP mask replies are never sent
IP fast switching is enabled
IP CEF switching is enabled
IP CEF switching turbo vector
IP Null turbo vector
IP multicast fast switching is enabled
IP multicast distributed fast switching is disabled
IP route-cache flags are Fast, CEF
Router Discovery is disabled
IP output packet accounting is disabled
IP access violation accounting is disabled
TCP/IP header compression is disabled
RTP/IP header compression is disabled
Probe proxy name replies are disabled
Policy routing is disabled
Network address translation is disabled
WCCP Redirect outbound is disabled
WCCP Redirect inbound is disabled
WCCP Redirect exclude is disabled
BGP Policy Mapping is disabled
Based on this output, what can be concluded?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "always"
Why it matters: Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
Clue: "never"
Why it matters: Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The MTU of 1400 bytes suggests this is a GRE or IPsec tunnel.
The tunnel interface is up/up with an MTU of 1400 bytes. The MTU is reduced from the default 1500, which is typical for GRE/IPsec tunnels to accommodate encapsulation overhead. This indicates the tunnel is likely a GRE or IPsec tunnel.
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 tunnel is down because the line protocol is up.
Why it's wrong here
The line protocol is up, so the tunnel is operational.
✓
The MTU of 1400 bytes suggests this is a GRE or IPsec tunnel.
Why this is correct
A reduced MTU is typical for tunnels that add encapsulation headers, such as GRE (24 bytes) or IPsec (up to 100 bytes).
Clue confirmation
The clue words "always", "never" in the question point toward this answer.
The tunnel is using IP compression because TCP/IP header compression is disabled.
Why it's wrong here
TCP/IP header compression is disabled, but that does not indicate compression is used; it is the opposite.
✗
The tunnel has a helper address configured for DHCP.
Why it's wrong here
The output shows 'Helper address is not set', so no DHCP helper is configured.
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
Command / output trap
The output shows 'Helper address is not set', so no DHCP helper is configured.
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 network engineer segments a warehouse floor into three subnets: 20 scanners, 5 printers, and 2 management hosts. Picking the wrong mask wastes addresses or leaves too few usable hosts. Exam questions test whether you can apply CIDR notation, calculate block size, and identify the correct usable-host range for a given prefix.
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 350-401 subnetting questions on CIDR, address ranges, and subnet selection.
VPN Technologies — This question tests VPN Technologies — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The MTU of 1400 bytes suggests this is a GRE or IPsec tunnel. — The tunnel interface is up/up with an MTU of 1400 bytes. The MTU is reduced from the default 1500, which is typical for GRE/IPsec tunnels to accommodate encapsulation overhead. This indicates the tunnel is likely a GRE or IPsec tunnel.
What should I do if I get this 350-401 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 350-401 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "always", "never". Absolute qualifier. An answer using 'always' is only correct if there are genuinely no exceptions — absolute statements are often wrong in networking.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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