Interface Shows Down/Down — Layer 1 Physical Issue
Presenting Symptom
The interface shows 'down/down' in the output of 'show interfaces' or 'show ip interface brief'.
Network Context
A small branch office with a Cisco 4321 router connected to an ISP modem via a copper Ethernet cable. The router runs IOS XE 16.9. The link between the router and modem is critical for internet access. The interface in question is GigabitEthernet0/0/0.
Diagnostic Steps
Check Interface Status
show interfaces GigabitEthernet0/0/0GigabitEthernet0/0/0 is down, line protocol is down
Both the physical layer (status) and data link layer (line protocol) are down. This indicates a Layer 1 issue, such as a cable problem, faulty transceiver, or the remote device being powered off.
Inspect Physical Connections
show interfaces GigabitEthernet0/0/0 | include media type|duplex|speedMedia type is RJ45 Duplex: auto, Speed: auto
Check if the media type matches the cable (e.g., RJ45 for copper). If the media type is not set correctly or shows 'unknown', the interface may not detect the cable. Also verify that duplex and speed are set to auto or match the remote device.
Check Interface Counters for Errors
show interfaces GigabitEthernet0/0/0 | include errors|CRC|runts|giants 0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 output errors, 0 collisions, 0 interface resetsIf there are many input errors (CRC, runts, giants), it suggests a faulty cable or interference. If no errors but still down/down, the issue is likely a complete physical break or the remote device is off.
Test with a Known Good Cable and Device
Replace the cable with a known good one and connect to a known working device. Then run 'show interfaces GigabitEthernet0/0/0' again.GigabitEthernet0/0/0 is up, line protocol is up (if the issue was the cable or remote device)
If the interface comes up with a different cable or device, the original cable or remote device is faulty. If it remains down/down, the router interface itself may be defective.
Root Cause
The Ethernet cable connecting the router to the ISP modem is damaged or loose, causing a loss of physical connectivity. The interface cannot detect a carrier signal, resulting in the down/down state.
Resolution
Verification
Run 'show interfaces GigabitEthernet0/0/0' and verify the output shows 'GigabitEthernet0/0/0 is up, line protocol is up'. Also check 'show ip interface brief' to confirm the interface status is 'up/up'.
Prevention
1. Use high-quality, shielded Ethernet cables, especially in environments with electromagnetic interference. 2. Regularly inspect and secure cable connections to prevent accidental disconnection. 3. Implement interface monitoring with SNMP to alert on interface state changes.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions where you must identify Layer 1 issues. The exam may present a 'show interfaces' output and ask you to determine the cause of a down/down state. Key fact: down/down indicates a physical layer problem, while up/down indicates a Layer 2 problem (e.g., no keepalive).
Exam Tips
Remember: 'down/down' = Layer 1 issue; 'up/down' = Layer 2 issue (e.g., encapsulation mismatch, no keepalive).
When troubleshooting, always start with 'show interfaces' to see the status and line protocol.
Common exam trap: An administratively down interface shows 'administratively down, line protocol is down' — not just 'down/down'.
Commands Used in This Scenario
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
show ip interface brief
Displays a summary of all IP interfaces on the device, including their IP address, status, and protocol state, used for quick verification of interface configuration and connectivity.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
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