Getting only 100 MBit/s on a fiber connection usually indicates a negotiation failure between the Optical Network Terminal (ONT) and your router, or the use of Cat5 cabling that cannot support Gigabit speeds. To resolve this, verify that all Ethernet cables are rated Cat5e or higher, check that the router’s WAN port is configured for auto-negotiation, and ensure the ONT is not in a limited speed mode due to configuration errors.
Verify Physical Cabling Standards
The most common reason for a drop from 1 GBit/s to 100 MBit/s is the physical layer limitation. Ethernet standards dictate that standard Cat5 cables can only reliably support 100 MBit/s. While some high-quality Cat5 cables might occasionally negotiate a Gigabit link, it is unstable and rare. You must ensure that every cable connecting your Optical Network Terminal (ONT) to your router, and from the router to your devices, is at least Cat5e (Category 5 enhanced) or Cat6.
Inspect the jacket of each cable for printed text indicating its category rating. If you see "Cat5" without the "e," replace it immediately. Additionally, check for physical damage such as sharp bends, crushed connectors, or exposed wires. A damaged cable may fail to maintain all eight wire pairs required for Gigabit Ethernet, forcing the connection to fall back to the four-pair Fast Ethernet standard.
Check Router Port Configuration and Hardware
Once cabling is confirmed, inspect the hardware ports on both the ONT and the router. Some older routers have multiple WAN ports where only one supports Gigabit speeds, while others default to 100 MBit/s if they detect an incompatible device. Ensure you are using the specific port labeled "WAN" or "Internet" which is typically Gigabit-capable. Avoid using secondary LAN ports for the upstream connection unless explicitly supported by the manufacturer.
Navigate to your router’s administrative interface to review the status of the WAN connection. Look for the link speed indicator. If it shows 100 Mbps, try disabling and re-enabling the port. Some routers allow manual speed setting; ensure this is set to "Auto" or "Auto-Negotiation" rather than a fixed 100 Mbps. Forcing a fixed speed can cause mismatches with the ONT, resulting in a fallback to the lowest common denominator.
Inspect Optical Network Terminal (ONT) Settings
The ONT, often provided by your internet service provider, acts as the bridge between the fiber optic line and your electrical network. In some cases, the ONT may be configured to limit output speed for testing purposes or due to a provisioning error. Access the ONT’s management interface, usually via a local IP address provided in the device documentation, and check the Ethernet port status.
Look for any active restrictions or "speed limit" features within the ONT settings. If available, ensure the Ethernet port is set to full duplex and auto-negotiation. If the ONT has a dedicated "Gigabit" switch or button, ensure it is enabled. Some enterprise-grade ONTs allow administrators to lock port speeds; if you suspect this, contact your ISP to verify the provisioning profile matches your subscribed tier.
Update Firmware and Drivers
Outdated firmware on your router or drivers on your computer can cause negotiation issues. Manufacturers frequently release updates that fix compatibility problems with new ONT models or improve Gigabit stability. Check the manufacturer’s website for the latest firmware version for your specific router model and apply it according to their instructions. Similarly, update the network adapter drivers on your PC to ensure proper communication with the router.
After updating, reboot both the ONT and the router to clear any cached negotiation states. This forces the devices to renegotiate the link parameters from scratch, often resolving transient handshake failures that result in reduced speeds.
Test with a Direct Connection
To isolate whether the issue lies with the router or the ONT, connect a laptop directly to the ONT using a known-good Cat5e or Cat6 cable. Disable Wi-Fi on the laptop and use an Ethernet port. Run a speed test directly from the laptop. If you achieve 1 GBit/s, the problem is likely with your router’s configuration or hardware. If you still get 100 MBit/s, the issue is with the ONT or the ISP line provisioning.
This step helps determine if you need to replace the router or contact your ISP for backend assistance. If the direct connection works, consider resetting your router to factory defaults and reconfiguring it, as complex custom settings may be interfering with the WAN negotiation process.
Frequently Asked Questions
Why does my speed test show 100 Mbps even though I pay for 1 Gbps?
This is almost always a physical layer issue, typically caused by using Cat5 cables or a faulty Ethernet port. Verify that all cables are Cat5e or higher and that the router’s WAN port is functioning correctly. If hardware is fine, the ONT may be misconfigured.
Can a bad Ethernet cable cause intermittent speed drops?
Yes, damaged or low-quality cables can cause packet loss and force the connection to downgrade to 100 Mbps to maintain stability. Always replace cables that show visible wear, kinks, or broken clips to ensure consistent Gigabit performance.
How do I check if my router supports Gigabit Ethernet?
Check the technical specifications on the router’s box or the manufacturer’s website for "Gigabit WAN" or "10/100/1000 Mbps" port descriptions. Older routers may only have Fast Ethernet (10/100 Mbps) ports, which physically cannot exceed 100 Mbps regardless of your internet plan.
What should I do if the ONT limits speed despite correct cabling?
Contact your Internet Service Provider to check the provisioning on their end. The ONT may be locked to a lower speed tier due to account settings or a software glitch that requires remote reset or replacement by a technician.
Does Wi-Fi affect the wired connection speed to the router?
No, Wi-Fi performance does not impact the wired Ethernet link between the ONT and the router. However, Wi-Fi congestion can affect the speed between your router and wireless devices. Test wired connections first to isolate the bottleneck.