easyMultiple Choice
300-410 Practice Question: Runs the following command on Router R5: R5# show…
A network engineer runs the following command on Router R5:
R5# show logging | include %LINEPROTO-5-UPDOWN *Mar 1 00:00:10.123: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up *Mar 1 00:00:20.456: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down *Mar 1 00:00:30.789: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up *Mar 1 00:00:40.012: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down *Mar 1 00:00:50.345: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up *Mar 1 00:01:00.678: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down
Based on this output, what is the most likely problem?
⚠ Common exam trap
300-410 often tests the interpretation of log messages; candidates may confuse administratively down with flapping or attribute flapping to non-physical causes.
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
✓
There is a physical layer issue causing the interface to flap.
The output shows the line protocol on GigabitEthernet0/0 repeatedly changing between up and down, which indicates interface flapping. This is typically caused by a physical layer issue such as a faulty cable, bad port, or duplex mismatch. The other options are not supported by the output: administratively down would show a different message, high bandwidth delay product does not cause flapping, and high CPU load would not directly cause line protocol state changes.
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 interface is administratively down.
Why it's wrong here
Administratively down produces a %LINK-3-UPDOWN or 'interface down' message and the line protocol stays down until 'no shutdown'. Here the line protocol repeatedly transitions up then down, indicating a physical or Layer 1/2 instability such as a flapping link. Administrative shutdown would be the cause if the interface never came up at all.
- ✓
There is a physical layer issue causing the interface to flap.
Why this is correct
Repeated up/down transitions of the line protocol on GigabitEthernet0/0 indicate interface flapping. Because the physical layer must stay stable for the line protocol to remain up, this recurring pattern points to a cabling, transceiver or duplex fault rather than a routing or configuration problem.
- ✗
The interface is configured with a high bandwidth delay product.
Why it's wrong here
A high bandwidth-delay product affects TCP windowing and throughput, not interface state. It cannot cause the line protocol to cycle up and down. BDP tuning is relevant when optimising long-fat-network throughput, whereas the log shows repeated link-state transitions pointing to physical instability such as a bad cable or duplex mismatch.
- ✗
The router is experiencing a high CPU load due to routing updates.
Why it's wrong here
High CPU from routing updates can delay processing but does not itself toggle the line protocol; the interface state is driven by physical and data-link events. The log shows repeated up/down transitions, characteristic of a flapping link. CPU load would be the suspect when routing adjacencies drop while interfaces remain up.
Quick reference
OSI Model Reference
| Layer | Name | PDU | Key Protocols / Devices |
|---|---|---|---|
| 7 | Application | Data | HTTP, HTTPS, DNS, SMTP, FTP, SSH |
| 6 | Presentation | Data | TLS / SSL, JPEG, ASCII encoding |
| 5 | Session | Data | NetBIOS, RPC, SIP |
| 4 | Transport | Segment / Datagram | TCP, UDP |
| 3 | Network | Packet | IP, ICMP, OSPF — Routers |
| 2 | Data Link | Frame | Ethernet, Wi-Fi, PPP — Switches, Bridges |
| 1 | Physical | Bits | Cables, NICs, Hubs, Repeaters |
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Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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