Set network interface speed with systemd-networkd

Sometimes automation is your best friend and sometimes it isn’t. Typically, when two devices are connected via ethernet cables, they negotiate the best speed they can manage across a network link. They also try to agree on whether they can run full or half duplex across the network link.

Most of the time, this works beautifully. It can break down with strange networking configs, damaged adapters, or finicky cables. In those situations, if you can rule out physical damage to any parts involved, you may need to disable autonegotiation to get the functionality you want.

Slow home network #

My home internet connection has a 400 megabit per second downlink, but noticed my downloads were slower this week and if someone was actively downloading something, such as an Xbox update, the internet latency shot up past 300ms. A few speed tests later and I found my internet speeds were limited to about 88 megabits per second.

My home firewall is a Dell Optiplex 7060 with an Intel I350-T2 dual-port gigabit ethernet card. My internet-facing NIC connects to a Netgear CM500-NAS cable modem. A quick check of my firewall’s external adapter revealed the problem:

# ethtool enp2s0f0 | grep Speed
	Speed: 100Mb/s

Well that’s certainly not right. The I350 is a gigabit card and the CM500-NAS is rated for speeds well over 100 mb/s. Rebooting the cable modem and the router itself didn’t change anything. I replaced the ethernet cable with a few others and there was no change there, either.

At this point, I was worried that my cable modem or adapter might have malfunction. At least there’s one more option.

Manually set the link speed #

Before we approach this section, here’s a reminder:

💣 Be sure that you have some other way to get into your system if manually setting the link speed fails.

If the system is remote to you, such as a dedicated server, virtual machine, or a faraway edge device that requires a 5 hour drive, you may want to consider other options. There’s a chance that manually setting the link speed may cause the link negotiation to fail entirely.

systemd-networkd gives you plenty of low-level control over network interfaces using the link files. These files have two parts:

• a [Match] section that tells systemd-networkd about the network devices that need special configuration
• a [Link] section that has special configuration for a network interface

We need two of these configurations in the [Link] section:

• BitsPerSecond= specifies the speed for the device with K/M/G suffixes
• Duplex= specifies half or full duplex

In this example, I’ll match the interface on its MAC address and set the speed/duplex:

# /etc/systemd/networkd/internet.link
[Match]
MACAddress=a0:36:9f:6e:52:26

[Link]
BitsPerSecond=1G
Duplex=full

We can apply the configuration change by restarting systemd-networkd:

systemctl restart systemd-networkd

Now let’s check the speeds:

# ethtool enp2s0f0 | grep Speed
	Speed: 1000Mb/s

Perfect! 🎉

Digging for answers #

Once the network speeds were working well again and my kids weren’t upset by glitches in their Netflix shows, I decided to look for issues that might be causing the negotiation to fail. Some quick checks of the network card show some potential issues:

# ethtool -S enp2s0f0 | grep error
     rx_crc_errors: 22
     rx_missed_errors: 0
     tx_aborted_errors: 0
     tx_carrier_errors: 0
     tx_window_errors: 0
     rx_long_length_errors: 0
     rx_short_length_errors: 0
     rx_align_errors: 0
     rx_errors: 33
     tx_errors: 0
     rx_length_errors: 0
     rx_over_errors: 0
     rx_frame_errors: 0
     rx_fifo_errors: 0
     tx_fifo_errors: 0
     tx_heartbeat_errors: 0

I’ve swapped network cables between the devices a few times and these errors continue to appear frequently. My I350 card is several years old and discontinued from Intel, so this could be the culprit. It’s also been moved between quite a few different computers over the years. A replacement with something new might be in my future.

Photo credit: Charles Deluvio on Unsplash