Lots of link up and down

[Zephyr]

during boot:

Code Select Expand


Configuring VLAN interfaces...em0: link state changed to DOWN
vlan0: changing name to 'em0_vlan20'
em0: link state changed to UP
em0_vlan20: link state changed to UP
done.
Configuring LAN interface... .em0: link state changed to DOWN
em0_vlan20: link state changed to DOWN
em0: link state changed to UP
em0_vlan20: link state changed to UP
em0: link state changed to DOWN
em0_vlan20: link state changed to DOWN
em0: link state changed to UP
em0_vlan20: link state changed to UP
em0: link state changed to DOWN
em0_vlan20: link state changed to DOWN
em0: link state changed to UP
em0_vlan20: link state changed to UP
em0: link state changed to DOWN
em0_vlan20: link state changed to DOWN
em0: link state changed to UP
em0_vlan20: link state changed to UP
em0: link state changed to DOWN
em0_vlan20: link state changed to DOWN
em0: link state changed to UP
em0_vlan20: link state changed to UP
done.
Configuring WAN interface...


Connected switch confirms this in the log.

How to troubleshoot?