5.2 Given a scenario, troubleshoot common cabling and physical interface issues
Interface Issues
📘CompTIA Network+ (N10-009)
Port Status in Networking
Every switch port in a network can have a status that indicates if it’s working correctly or if there’s an issue. Monitoring port status is critical because it tells network administrators whether devices connected to the network can communicate properly.
There are several possible port statuses. For this section of the exam, you need to know Error-disabled, Admin down, and Suspended.
1. Error-disabled (Err-Disabled)
Definition:
A port becomes error-disabled when the switch detects a serious problem on that port. The switch automatically shuts the port to prevent network issues. Think of it as a safety mechanism.
Common causes:
- BPDU Guard violation: If someone connects a device that sends BPDUs (Bridge Protocol Data Units) on a port where it’s not allowed.
- Port security violation: If more devices are connected than allowed, or an unknown MAC address is detected.
- Duplex mismatch: When one side of the link is full-duplex and the other side is half-duplex, causing collisions.
- Loop or broadcast storm: Excessive broadcast traffic can trigger this.
Effect:
- The port stops forwarding traffic.
- Connected devices cannot communicate through this port.
- The port needs manual intervention or automated recovery to re-enable it.
How to fix:
- Identify the cause using commands like
show interfaces statusorshow running-config. - Correct the problem (e.g., adjust port security, fix cabling, or adjust duplex settings).
- Re-enable the port with a command like:
shutdown no shutdown - Verify with
show interfaces statusthat the port is back online.
Example in IT environment:
A network admin notices that a server cannot connect to the network. Checking the switch shows the port is error-disabled because the server’s network card has a MAC address not allowed by port security. The admin updates the allowed MAC list and re-enables the port.
2. Admin Down
Definition:
A port is Admin down when it has been manually disabled by the network administrator. This is different from error-disabled because it’s intentional, not triggered by a network problem.
Common causes:
- Admin shuts down the port to prevent unauthorized access.
- A port is not currently in use, so it’s administratively turned off to save resources or increase security.
Effect:
- Port does not pass any traffic.
- Devices connected to the port cannot communicate.
- Port will remain down until the admin manually enables it.
How to fix:
- If the port should be active, the admin enables it with:
no shutdown - Verify using
show interfaces status.
Example in IT environment:
An admin disables a switch port in a meeting room because it’s temporarily not needed. Later, when a new workstation is installed, the port is re-enabled.
3. Suspended
Definition:
A port is suspended when the switch temporarily disables the port because it has detected excessive or potentially harmful traffic. Suspended is less common than error-disabled or admin down, but it is important for security and network stability.
Common causes:
- Port security violation when violation mode is set to restrict or protect.
- Spanning Tree Protocol (STP) may put a port in a suspended state to prevent loops.
Effect:
- Port stops forwarding traffic temporarily.
- Some switches may automatically re-enable the port after a timeout (depending on configuration).
How to fix:
- Identify the cause of suspension using commands like:
show port-security show spanning-tree - Resolve the issue (e.g., reduce security violations or fix loop issues).
- If needed, manually re-enable the port.
Example in IT environment:
A switch detects a device that keeps sending frames with an unknown MAC address repeatedly. The port is suspended to prevent a security breach. Once the admin investigates and authorizes the device, the port can be reactivated.
Summary Table
| Port Status | Cause | Effect | Fix |
|---|---|---|---|
| Error-disabled | Serious problem detected (BPDU, security, duplex mismatch) | Port stops forwarding traffic | Identify cause → fix → shutdown/no shutdown |
| Admin down | Port manually disabled by admin | Port remains inactive | no shutdown to re-enable |
| Suspended | Security or STP issues detected | Port temporarily stops traffic | Identify cause → fix → re-enable |
Key Exam Tips:
- Recognize the difference:
- Error-disabled: automatic shutdown due to problem.
- Admin down: manual shutdown by admin.
- Suspended: temporary shutdown due to detected issue.
- Know commands for verification:
show interfaces status show running-config show port-security show spanning-tree - Know commands for re-enabling ports:
interface [port-id] shutdown no shutdown - Remember security implication:
Many of these states are triggered to protect the network from loops, unauthorized access, or misconfigurations.
