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Cisco Nexus 9000 Series NX-OS Troubleshooting Guide, Release 10.4(x)

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The following is a troubleshooting guide for Cisco Nexus 9000, Release 10.4

Overview

Before a Cisco Nexus switch can forward frames between networks, each interface needs to be correctly configured. Interfaces can be:

  • Ethernet interfaces (physical ports)
  • VLAN interfaces (SVIs)
  • Management interface (mgmt0)

Each interface has two types of settings:

  • Administrative configuration (what you set up and control)
  • Operational status (current, read-only condition of attributes like speed)

Guidelines & Limitations

  • Check module status: Ensure all modules in the chassis are functioning correctly. Use show module to confirm that each module is active.
  • Port group mode: Only one port in a group of four can be in dedicated mode; the others become unusable.
  • No licensing is required for configuring ports.

Initial Troubleshooting Checklist

  • Check physical media: Ensure no damaged cables or connectors.
  • Verify SFPs: Use only Cisco-authorized SFPs and confirm they aren’t defective.
  • Enable the port: Use no shutdown command.
  • Check interface state: Use show interface to see if the port is up or down.
  • Confirm dedicated port configuration: If the port is dedicated, do not connect to the other three ports in the same group.

Viewing Port Information

Use show interface counters to check port counters. For more accurate troubleshooting:

  • Clear counters first to reset baseline data:
clear counters interface all
  • Look for mismatches between received and transmitted frames.

Additional commands:

show interface status
show interface capabilities
show udld
show tech-support udld

CLI-Based Troubleshooting of Port Statistics

Key Command: show interface

Provides details on speed, VLAN status, frames sent/received, and errors.

Example:

switch# show interface ethernet 2/45
Ethernet2/45 is down (Administratively down)
Hardware is 10/100/1000 Ethernet, ...
0 input error 0 short frame 0 watchdog

This command helps check if the port is shut down, physically disconnected, or has encountered errors.

Common Port Interface Issues

1. The Interface Configuration Disappeared

Cause: Switched from Layer 2 (switchport) to Layer 3 (routed) mode or vice versa.
Solution: Cisco NX-OS removes the interface config when changing port modes. Reconfigure the interface after switching modes.

2. Cannot Enable an Interface

Possible Causes & Solutions:

  • Dedicated port group issue: Check the port’s rate mode with show running-config interface.
  • Incompatible configuration: Use show interface capabilities to match settings.
  • Missing or suspended VLAN: Use show vlan brief and reactivate if needed.
  • Incorrect SFP: Check with show interface brief and replace with a Cisco-supported SFP if necessary.

3. Cannot Configure a Dedicated Port

Causes & Solutions:

  • Other ports in the group are active: Shut down the other three ports first.
  • Not the first port in the group: Only the first port (e.g., 1/1 in a group of 1/1–1/4) can be set to dedicated.

4. Port Stays in "Link Failure" or "Not Connected"

Causes & Solutions:

  • Bad port connection: Verify cable type. Try shutdown / no shutdown or move the cable to a different port.
  • Transit fault in the SFP: Check seating. Replace if needed.
  • Link stuck initializing: Check logs with show logging. Reset port if needed.

5. Unexpected Link Flapping

Causes & Solutions:

  • Excessive bit rate triggers ErrDisabled: Reset port with shutdown / no shutdown.
  • Hardware/software errors: Check logs with show logging.

6. Port Is in the ErrDisabled State

Cause: Often triggered by flapping port or hardware/media issue.
Solution:

  • Use show interface interface slot/port to confirm errDisabled status.
  • Verify cables, SFPs, and connections.
  • Check logs with show logging logfile for specific error messages.

In Summary:

  • Always verify hardware (modules, ports, cables, and optics) before deeper troubleshooting.
  • Use show interface, show interface counters, and show logging logfile to find the root cause.
  • Keep in mind port grouping restrictions (dedicated vs. shared).
  • ErrDisabled ports usually indicate repeated problems (link flaps, incompatible settings, or hardware faults).
  • Changing a port from Layer 2 to Layer 3 (or vice versa) wipes its configuration. Reconfigure as needed after switching modes.

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Document Information

Modified date:
04 March 2025

UID

ibm17184764

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