Troubleshooting and Solutions for Optical Module Transmission Faults

This type of fault mainly includes ports not coming UP, ports showing an UP status but not transmitting or receiving packets, frequent port up/down events, and CRC errors.
This article analyzes these common issues in detail.

I. Port Does Not Come UP

Taking 10G SFP+/XFP optical modules as an example, when an optical port fails to come UP after being interconnected with another device, troubleshooting can be carried out from the following five aspects:

Step 1: Check whether the speed and duplex modes on both ends match

Execute the show interface brief command to view the port status.
If there is a mismatch, configure the port speed and duplex mode using the speed and duplex commands.

Step 2: Check whether the device port and optical module match in speed and duplex mode

Use the show interface brief command to verify the configuration.
If there is a mismatch, configure the correct speed and duplex mode using the speed and duplex commands.

Step 3: Check whether both ports are functioning properly

Use a loopback test to verify whether both ports can come UP.

• On 10G SFP+ ports on the line card, use a 10G SFP+ direct attach cable (for short-distance connections) or SFP+ optical modules with fiber patch cords.

• On 10G XFP ports, use XFP optical modules and optical fiber for testing.

If the port comes UP, the peer port is abnormal.
If the port does not come UP, the local port is abnormal.
The issue can be verified by replacing the local or peer port.

Step 4: Check whether the optical module is functioning properly

Mainly check DDM information, optical power, wavelength, and transmission distance.

• DDM Information
  Use the show interfaces transceiver detail command to check whether parameters are normal.
  If alarms appear, the optical module may be faulty or incompatible with the optical interface type.

• Optical Power
  Use an optical power meter to test whether the transmit and receive optical power levels are stable and within the normal range.

• Wavelength / Distance
  Use the show transceiver interface command to verify whether the wavelength and transmission distance of the optical modules on both ends are consistent.

Step 5: Check whether the optical fiber is normal

For example:

• Single-mode SFP+ optical modules must be used with single-mode fiber.

• Multimode SFP+ optical modules must be used with multimode fiber.

If there is a mismatch, replace the fiber with the appropriate type immediately.

If the fault cannot be located after completing all the above checks, it is recommended to contact the supplier’s technical support personnel for assistance.

II. Port Status Is UP but Not Transmitting or Receiving Packets

When the port status is UP but packets cannot be transmitted or received, troubleshoot from the following three aspects:

Step 1: Check packet statistics

Check whether the port status on both ends remains UP and whether packet counters on both ends are increasing.

Step 2: Check whether port configuration affects packet transmission

• First, check whether any network configuration has been applied and verify whether it is correct. If necessary, remove all configurations and test again.

• Second, check whether the port MTU value is 1500. If the MTU is greater than 1500, modify the configuration accordingly.

Step 3: Check whether the port and link medium are normal

Replace the connected port and connect it to another port to see whether the same issue occurs.
If the problem persists, replace the optical module.

If the issue cannot be resolved after the above checks, it is recommended to contact the supplier’s technical support personnel.

III. Port Frequently Goes UP or DOWN

When an optical port frequently goes UP or DOWN:

• First, confirm whether the optical module is abnormal by checking its alarm information, and troubleshoot both optical modules and the connecting fiber.

• For optical modules that support digital diagnostic monitoring, check the DDM information to determine whether the optical power is at a critical threshold.

  • If the transmit optical power is at a critical value, replace the optical fiber or optical module for cross-verification.

  • If the receive optical power is at a critical value, troubleshoot the peer optical module and the connecting fiber.

When this issue occurs with electrical optical modules, try configuring the port speed and duplex mode.

If the problem persists after checking the link, peer devices, and intermediate equipment, it is recommended to contact the supplier’s technical support personnel.

IV. CRC Errors

Step 1: Check packet statistics to identify the issue

Use the show interface command to check error packet statistics in both ingress and egress directions and determine which counters are increasing.

• CEC, frame, or throttles errors increasing on ingress

  • Use testing instruments to check whether the link is faulty. If so, replace the network cable or optical fiber.

  • Alternatively, connect the cable or optical module to another port.

    • If errors reappear after changing ports, the original port may be faulty.

    • If errors still occur on a known-good port, the issue is likely with the peer device or the intermediate transmission link.

• Overrun errors increasing on ingress
  Run the show interface command multiple times to check whether input errors are increasing.
  If so, this indicates increasing overruns, possibly caused by internal congestion or blockage within the line card.

• Giants errors increasing on ingress
  Check whether jumbo frame configurations on both ends are consistent, including:

  • Default maximum packet length

  • Allowed maximum packet length

Step 2: Check whether the optical module power is normal

Use the show transceiver interfaces detail command to check the current digital diagnostic values of the installed optical module.
If the optical power is abnormal, replace the optical module.

Step 3: Check whether the port configuration is normal

Use the show interface brief command to verify the port configuration, focusing on:

• Negotiation status

• Duplex mode

• Port speed

If half-duplex mode or speed mismatch is found, configure the correct duplex mode and port speed using the duplex and speed commands.

Step 4: Check whether the port and transmission medium are normal

Replace the connected port to see whether the issue persists.
If it does, check intermediate devices and transmission media.
If they are normal, replace the optical module.

Step 5: Check whether the port is receiving a large number of flow control frames

Use the show interface command to check the pause frame counter.
If the counter continues to increase, the port is sending or receiving a large number of flow control frames.

Also check whether ingress and egress traffic is excessive and whether the peer device has sufficient traffic processing capability.

If no problems are found with configuration, peer devices, or the transmission link after completing all checks, please contact the supplier’s technical support team directly.