Why do we see the message "speed changed to 0 for port <NIC name>" in dmesg and /var/log/messages?

Issue

These messages are printed by LACP bonding module when it detects a link going down or flapping. The system may lose all network connectivity over LACP bond or over a particular bond NIC when this happens. Occasionally, despite the link showing as UP in ethtool, the system drops all network access.

Similar message displayed in dmesg:

• For RHEL 7 and earlier versions:

[NNNNN.NNNNN] [NIC model] 0000:03:00.0 [NIC name here] speed changed to 0 for port [NIC name here]

• For Later versions:

[NNNNN.NNNNN] [NIC model] 0000:03:00.0 [NIC name here] speed changed to 0 on port [NIC name here]

Apparently, the message occurs with multiple NIC vendors, and isn't limited to RHEL. There is a possibility of dropped packets accompanying the message, but this is not always the case.