Use this guide to diagnose and resolve Ethernet port link failures on Smart Access Gateway (SAG) devices.
Symptoms
The Ethernet port shows no link.
Causes and solutions
Work through each possible cause in order. Once you identify the cause, apply the corresponding solution and verify that the port comes up.
| Possible cause | Solution |
|---|---|
| Device not powered on, or cable not fully seated | Make sure both the local and peer SAG devices are powered on. Unplug the cable and firmly reseat it into the Ethernet port on both ends. |
| Cable or port module faulty | Swap the cable with a known-good cable. Swap the port module as well. |
| Cable too long or excessive optical loss | See Twisted pair cable checks or Optical fiber checks below. |
| Port or SAG device hardware failure | Check whether the local or peer SAG device is faulty. If the issue persists, contact Alibaba Cloud support. |
Twisted pair cable checks
If your devices are connected by a twisted pair cable, run the following checks in order.
1. Test the cable
Use a cable tester to verify the cable is working. If the cable is faulty, replace it.
2. Check cable length
The cable between the two devices must not exceed 100 meters. 10/100/1000 Mbit/s electrical ports use RJ45 connectors, and Category 5 or later cables support a maximum transmission distance of 100 meters.
If the cable exceeds 100 meters:
Reposition the devices to shorten the run.
If repositioning is not possible, insert a repeater, hub, or Ethernet switch between the SAG and peer devices to extend the distance.
3. Verify cable type
Use the correct cable type based on the devices you are connecting.
| Connection | Cable type |
|---|---|
| Router to Ethernet switch | Straight-through |
| Router to hub | Straight-through |
| Computer to Ethernet switch | Straight-through |
| Computer to hub | Straight-through |
| Router to router | Crossover |
| Router to computer | Crossover |
| Ethernet switch to Ethernet switch | Crossover |
| Hub to Ethernet switch | Crossover |
| Hub to hub | Crossover |
| Computer to computer | Crossover |
Optical fiber checks
If your devices are connected by an optical fiber, run the following checks in order.
1. Match the optical module to the fiber type
The optical module must match the optical fiber:
A multimode optical fiber requires a multimode optical module.
A single-mode optical fiber requires a single-mode optical module. A single-mode optical module does not work with a multimode optical fiber.
The wavelength of both connected optical modules must be the same.
To identify fiber type by color: single-mode fibers are typically yellow; multimode fibers are typically orange.
If there is a mismatch, replace the optical module or fiber so that they match.
2. Check fiber length against module specs
The optical fiber length must be within the transmission distance supported by the optical module. If the fiber is too long, either shorten it or replace the optical module with one that supports a longer transmission distance.
3. Measure signal attenuation
Use a tester to measure signal attenuation. If attenuation exceeds the valid range:
Replace the optical fiber.
If the issue persists after replacing the fiber, shorten the fiber length.
4. Test the fiber and optical module
Run either of the following tests to isolate a faulty cable or module:
Tester: Use a cable tester to verify the fiber is working.
Physical loopback test: Connect both ends of the fiber to the same optical module. If the port comes up, the fiber is working. If the port does not come up, the fiber is faulty.
If the fiber is faulty, replace it. If the issue persists after replacing the fiber, replace the optical module.
Escalate to support
If none of the above steps resolve the issue, the port or SAG device hardware may be faulty. Contact Alibaba Cloud support for further diagnosis.