10GbE Troubleshooting: Cables, SFP, MTU and iperf3

A 10 Gb connection that only delivers 1 Gb speeds, drops out, or shows the right speed but copies slowly is a common problem in edit suites. It almost always comes down to one of five things: the cable, the transceiver, the switch port, the settings on the two ends, or the computer itself. This guide works through them in an order that finds the fault quickly, and ends with a test that proves the result.

Tools and menu names here follow the documentation for Synology DSM 7.x, macOS and Windows 11, and iperf3. Switch menus vary by brand and model, so look for the same setting names in yours.

The quickest first thing to try

Swap the cable for a different one that you know is good, and plug into a different port on the switch. Cables are a frequent cause. Then look at the link speed that the computer and the switch each report. If both say 10 Gb, go to the iperf3 test. If one says less, the cable, transceiver or port is the suspect.

Step by step

1. 1Check the reported speed at both ends. On Windows, Settings, Network and internet, Ethernet shows the link speed. On a Mac, System Settings, Network, your Ethernet service, Details, Hardware shows speed and MTU in recent versions of macOS. On the NAS and the switch, use the status page for the port. A mismatch between the two ends is a clue.
2. 2Check the cable type and length. For copper 10GBASE-T, Cat6a is the cable that supports the full 100 m run. Cat6 is commonly quoted as good for about 55 m, and the industry guidance for installed Cat6 depends on crosstalk conditions, so shorter runs are safer. Cat5e is not a good choice at 10 Gb. Read the printing on the cable jacket, and avoid cables that are damaged, kinked or crushed behind desks. Category numbers on a cheap cable are not proof that it performs as labelled.
3. 3Look for a negotiated-down link. Many 10GBASE-T devices can also run at 5, 2.5 or 1 Gb. When a link drops to a lower speed, a marginal cable, a long run or a bad connector is the usual reason. Replace the cable first, then re-seat the connector.
4. 4For SFP+ ports, check the transceiver. A direct attach copper cable (DAC) is meant for short runs between nearby devices. Optical modules must match each other and the fiber: short-reach (SR) modules are for multimode fiber, long-reach (LR) modules are for single-mode. Wavelengths must match at both ends. If a fiber link will not come up, try swapping the transmit and receive strands on a duplex pair, and clean the connector ends. Some switches and network cards accept only modules that they recognize, so check the vendor's compatibility information before you buy. 10GBASE-T modules in SFP+ ports can run warm, so check airflow.
5. 5Check the switch. Confirm the port is enabled at the expected speed, with auto-negotiation on, in the right VLAN, and not part of a mismatched link aggregation group. Synology notes that SMB3 Multichannel cannot be combined with link aggregation, so a team using both may see odd behavior. Check the uplink between switches too: a 10 Gb NAS behind a 1 Gb link to the editing room runs at 1 Gb for everyone beyond it.
6. 6Check the computer. A card in a PCIe slot that provides too few lanes, a USB or Thunderbolt adapter that is not sized for 10 Gb, or an old driver can all limit speed. Update the driver and firmware, and try a different slot or adapter. Our comparison of Thunderbolt and 10GbE explains the adapter options.
7. 7Check MTU. MTU is the largest packet size on the network. The standard is 1500 bytes. Jumbo frames, commonly 9000, can reduce overhead, but only when every device in the path uses the same value. Synology states that all computers and devices that access the NAS must support jumbo frames and use the same MTU value. On Synology, the setting is in Control Panel, Network, Network Interface. On the switch, jumbo frames must be enabled on the ports and VLAN in use.
8. 8Test the MTU with a ping that cannot be fragmented. For a 9000 byte MTU, send a payload of 8972 bytes (9000 minus 20 for IP and 8 for ICMP). On Windows: ping -f -l 8972 followed by the NAS address. On a Mac: ping -D -s 8972 followed by the NAS address. If you get replies, jumbo frames work along the path. If you see a message that the packet needs to be fragmented, some device in the path is still at 1500.
9. 9Measure with iperf3. Install iperf3 on the computer and on another machine near the NAS, or on the NAS if it supports it. On the receiving side run iperf3 -s. On the sending side run iperf3 -c followed by the receiver address and -t 30. Add -P 4 to run four parallel streams, and -R to run in reverse, so the server sends and the client receives. Test both directions. These options are documented in the iperf3 manual.
10. 10Compare iperf3 with a real file copy. If iperf3 reaches close to link speed and a file copy is slow, the network is healthy and the problem is the NAS disks, SMB settings or the computer. See our Premiere dropped frames article.

What causes slow 10GbE

• Marginal copper cable or a long Cat6 run.
• A transceiver that does not match its partner or the fiber.
• A switch uplink that is slower than the NAS port.
• MTU set to 9000 on some devices and 1500 on others.
• A link-aggregation setting on one side only.
• A computer slot or adapter that cannot sustain 10 Gb.

What not to do

• Do not enable jumbo frames on the NAS alone. A mismatch can make large transfers fail while small ones work.
• Do not change MTU during working hours. Changing it can interrupt network connections.
• Do not trust a link light as proof of speed. Read the reported speed.
• Do not judge the network by a single file copy. Use iperf3, then copy files.
• Do not buy new switches before you have swapped the cable and run iperf3.

When it is time to call someone

If iperf3 is healthy and the cable and ports test clean but a workstation still cannot reach the speed it should, the problem is in the storage design, the switch configuration or a wider network issue. It is also time to call if you are planning a new 10GbE or faster install, and want the cable paths, switch and NAS ports planned properly. Our guide to small studio network setup shows what a sound design looks like.

Want a person to look at it?

This is the kind of problem our production consulting team handles: testing the cabling, switch and NAS settings together and fixing what is limiting your speed. See our network setup service, or tell us what is happening in your edit bay and a real person on our team will reply.