How is the "cardSpeed" value derived in the Network plugin (Linux OS)?

Assuming that the network sampler is configured not to use NIC tool, then netprobe will fetch the data from/sys/class/net/<interface>/[speed/duplex/carrier]. If these files are not accessible then it will fetch the additional data from the system commands ethtool or mii-tool which requires root access.

For example:

then the data is taken from: