How to Measure and Optimize Linux NIC Bandwidth Using ethtool, dmesg, and iperf3

1. Network Bandwidth

1.1 Concept

Network bandwidth is the amount of data that can be transferred per second, analogous to the number of lanes on a highway; more lanes mean higher capacity. It is a key metric for network planning, ISP selection, and performance evaluation.

1.2 Calculation

Bandwidth is measured in bits (b) while speed is measured in bytes (B). Common units are Kb, Mb, Gb for bits and KB, MB, GB for bytes. The basic conversion is 1 byte = 8 bits. Example conversions: 1Kb = 1024b, 1KB = 1024B, and so on. Bandwidth and speed share a factor of 1024, so speed = bandwidth / 8.

2. NIC Bandwidth

NIC bandwidth is the per‑second data rate a network card can receive or transmit, expressed in Mbps.

2.1 Check NIC bandwidth with ethtool

Install the utility, find the interface name (e.g., enp129s0f0), then run the following commands:

sudo apt install ethtool

ip a

ethtool enp129s0f0

The output line Speed: 1000Mb/s shows the NIC bandwidth. To display only the speed, use:

ethtool enp129s0f0 | grep -i speed

2.2 Check NIC bandwidth with dmesg

Use the kernel ring buffer to locate the NIC link status: dmesg | grep enp129s0f0 | grep up Typical output: igb: enp129s0f0 NIC Link is Up 1000 Mbps Full Duplex.

2.3 Check NIC bandwidth with mii‑tool

Install net‑tools and run:

sudo apt install net-tools

sudo mii-tool -v enp129s0f0

The line negotiated 1000baseT-FD indicates a 1 Gbps NIC.

3. Virtual Machine NIC Bandwidth

VM NIC bandwidth is limited by the underlying physical NIC, but virtual adapters such as VMXNET3 can advertise higher speeds (e.g., 10 Gbps) even when the host NIC is only 1 Gbps. The virtual switch handles intra‑host traffic without involving the physical NIC, while traffic leaving the host still respects the physical NIC limits.

4. Measure Real Network Bandwidth with iperf3

Install iperf3 on the server: yum install -y iperf3 Start the server: iperf3 -s Run the client from another host: iperf3 -c SERVER_IP iperf3 reports throughput, latency, jitter and packet loss. Example results show real throughput around 935 Mbits/s for two 1 Gbps NICs in the same rack, about 913 Mbits/s across different racks, and around 867 Mbits/s when a VM (hosted on a 1 Gbps NIC) communicates with a bare‑metal server.

5. Identify Network Bottlenecks

Determine the data path from client NIC through routers, switches, firewalls, and server NIC. The overall throughput is limited by the smallest link in the chain (the “bucket” principle). In a LAN where all devices have 1 Gbps NICs but the ISP link is 100 Mbps, the maximum end‑to‑end speed is 100 Mbps (≈12.5 MB/s).