Unusually slow uplink performance appears exclusively when the modem connects to an SFP+ module
Unusually slow uplink performance appears exclusively when the modem connects to an SFP+ module
Initially I want to describe how my home network is arranged. I’m using a 5G modem that links to the internet from a distant location—a ZTE MC801a offering up to 600Mbps download and 60Mbps upload, with two 1 Gbps RJ45 ports on it. About 10 meters away, there’s an unmanaged switch connected to the network. This switch features four 2.5Gbps ports plus two SFP+ 10Gbps modules. It has three ports currently in use, serving as the primary gateway for the whole network. The active ports are: two 2.5Gbps connections and one 10Gbps SFP+ port.
Specifically:
- Two of the four 2.5Gbps ports
- One 2x SFP+ 10Gbps module
The switch is the main entry point. The PCs connected are:
- My 2.5Gbps PC network card via 2.5Gbps RJ45
- Another 2.5Gbps access point
- The 1Gbps 5G modem using a 10m Cat5e cable
In short, the upload speed only reaches about 20Mbps when the modem connects to one of the two 10Gbps SFP+ ports, not the full 60Mbps. Download works fine. The odd part is that when I manually adjust the PC’s speed from 2.5Gbps down to 1Gbps on my desktop, the upload speed returns to normal.
I’ve tested several things:
- Switched modem to the 2.5Gbps RJ45 port – problem solved
- Used different operating systems; issue appears on both Linux and Windows
- Ran a speed test from the 2.5Gbps AP – no connection detected
- Reset all devices, changed Ethernet power settings – no improvement
- Experimented with shorter cables; 10m Cat5e to both PC and modem worked
Another laptop with a 1Gbps NIC (like the modem) connected to an SFP+ port showed normal results using iperf.
This suggests a compatibility issue possibly between the PC card and the router, but only when the switch is linked to the SFP+ 10Gbps port. Other devices seem unaffected. It’s puzzling and I’m still trying to figure out what’s happening.