Quick Answer
The answer is that the tunnel is a GRE tunnel that is up and operational. This is confirmed by the "Tunnel0 is up, line protocol is up" status, the "Encapsulation TUNNEL" line, and the "Tunnel protocol/transport GRE/IP" output, which together indicate a functional Layer 3 GRE tunnel with correctly configured source and destination IP addresses. On the ENCOR 350-401 exam, this "GRE tunnel show interfaces interpretation" tests your ability to distinguish between tunnel states—specifically, that "up/up" means the tunnel is logically and physically ready to forward traffic, even if no data is flowing, as shown by the zero packet counters. A common trap is assuming zero traffic means a misconfiguration; instead, remember that a tunnel can be fully operational but idle. Memory tip: "UP/UP = ready to go, even if no show."
This 350-401 practice question tests your understanding of wan 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 R4:
R4# show interfaces tunnel 0
Tunnel0 is up, line protocol is up Hardware is Tunnel Internet address is 10.0.0.4/30 MTU 17916 bytes, BW 100 Kbit/sec, DLY 50000 usec, reliability 255/255, txload 1/255, rxload 1/255
Encapsulation TUNNEL, loopback not set
Keepalive not set Tunnel source 192.168.1.4, destination 192.168.2.4 Tunnel protocol/transport GRE/IP Key disabled, sequencing disabled Checksumming of packets disabled Last input never, output never, output hang never Last clearing of "show interface" counters never Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo Output queue: 0/0 (size/max) 5 minute input rate 0 bits/sec, 0 packets/sec 5 minute output rate 0 bits/sec, 0 packets/sec 0 packets input, 0 bytes, 0 no buffer Received 0 broadcasts (0 IP multicasts) 0 runts, 0 giants, 0 throttles 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort 0 packets output, 0 bytes, 0 underruns 0 output errors, 0 collisions, 0 interface resets 0 unknown protocol drops 0 output buffer failures, 0 output buffers swapped out
Based on this output, what is true about this tunnel?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
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.
The tunnel is using IPsec encryption.
Why wrong: The encapsulation is GRE/IP, not IPsec.
The tunnel is a GRE tunnel that is up and operational.
The interface is up/up and protocol is GRE/IP.
The tunnel is using MPLS over GRE.
Why wrong: No MPLS is indicated; it is plain GRE.
The tunnel is down because there are no packets.
Why wrong: The tunnel is up/up; lack of traffic does not mean it is down.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
The tunnel is a GRE tunnel that is up and operational.
The tunnel is up/up, uses GRE/IP encapsulation, and has a source and destination IP. The lack of traffic is indicated by zero packets input/output.
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
For each option: why learners choose it and why it is or isn't the right answer here.
The tunnel is using IPsec encryption.
Why it's wrong here
The encapsulation is GRE/IP, not IPsec.
The tunnel is a GRE tunnel that is up and operational.
Why this is correct
The interface is up/up and protocol is GRE/IP.
Clue confirmation
The clue word "never" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
The tunnel is using MPLS over GRE.
Why it's wrong here
No MPLS is indicated; it is plain GRE.
The tunnel is down because there are no packets.
Why it's wrong here
The tunnel is up/up; lack of traffic does not mean it is down.
Common exam traps
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
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.
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
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.
What to study next
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.
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FAQ
WAN Technologies — This question tests WAN Technologies — CIDR notation defines the prefix length..
The correct answer is: The tunnel is a GRE tunnel that is up and operational. — The tunnel is up/up, uses GRE/IP encapsulation, and has a source and destination IP. The lack of traffic is indicated by zero packets input/output.
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.
Yes — watch for: "never". Absolute qualifier. True only if the statement has zero exceptions — be cautious of options that seem obvious but break down in edge cases.
CIDR notation defines the prefix length.
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Last reviewed: Jun 18, 2026
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