Troubleshooting Network Interface Speed and Duplex Issues
Interface issues are typically diagnosed by examining interface status (up/down) and checking for input/output errors indicating speed or duplex mismatches.
Interpreting Interface Status Codes
The `show ip interface brief` command provides Layer 1 and Layer 2 status. - **Up / Up:** The link is fully operational. - **Down / Down:** A physical issue (bad cable, powered off device, or no SFP). - **Up / Down:** A Layer 2 issue, such as a clock rate mismatch (on serial links) or an encapsulation mismatch (e.g., HDLC vs. PPP). - **Administratively Down:** The interface was manually disabled with the `shutdown` command.
Duplex Mismatches and Collisions
If one side of a link is Full-Duplex and the other is Half-Duplex, the link will stay up, but performance will degrade severely. The Half-Duplex side will register collisions, while the Full-Duplex side will register FCS (Frame Check Sequence) errors and runts. Check statistics using `show interfaces` and look for late collisions.
Speed Mismatches and Auto-Negotiation
Auto-negotiation usually handles speed and duplex settings. However, if one side is hardcoded (e.g., 100 Mbps) and the other is auto, the auto side will detect the speed but default to half-duplex, causing a mismatch. It is best practice to either rely fully on auto-negotiation or hardcode settings identically on both sides.
❓ Frequently Asked Questions
What causes an interface to show 'Administratively Down'?
This means the interface has been manually disabled by an administrator using the 'shutdown' command.
What do late collisions indicate on a switch port?
Late collisions usually indicate a duplex mismatch or a cable that exceeds Ethernet length limitations.
How can you view interface error counters?
The 'show interfaces' command displays detailed statistics, including input errors, output errors, runts, giants, and collisions.