Repeated link down/up when using DHCP server on LAN

I have a windows 11 Home PC connected on eth3 and logread prints the following output:

Jun 20 10:08:55 kernel: [ 493.761371] nss-dp 3a001600.dp4 eth3: PHY Link up speed: 100
Jun 20 10:08:55 kernel: [ 493.761445] br-lan: port 4(eth3) entered blocking state
Jun 20 10:08:55 kernel: [ 493.766108] br-lan: port 4(eth3) entered forwarding state
Jun 20 10:08:55 netifd: Network device 'eth3' link is up
Jun 20 10:08:56 kernel: [ 494.785311] nss-dp 3a001600.dp4 eth3: PHY Link is down
Jun 20 10:08:56 kernel: [ 494.785688] br-lan: port 4(eth3) entered disabled state
Jun 20 10:08:56 netifd: Network device 'eth3' link is down
Jun 20 10:08:57 kernel: [ 495.809339] nss-dp 3a001600.dp4 eth3: PHY Link up speed: 100
Jun 20 10:08:57 kernel: [ 495.809438] br-lan: port 4(eth3) entered blocking state
Jun 20 10:08:57 kernel: [ 495.814072] br-lan: port 4(eth3) entered forwarding state
Jun 20 10:08:57 netifd: Network device 'eth3' link is up
Jun 20 10:09:01 zebra[5003]: [WPPMZ-G9797] if_zebra_speed_update: eth3 old speed: 4294967295 new speed: 100

Does anybody know why this output prints repeatedly link is up/down ?

please post the output of ubus call system board.

{
	"kernel": "5.4.89",
	"hostname": "---",
	"model": "--- RAX3600YCXT",
	"board_name": "RAX3600YCXT",
	"release": {
		"distribution": "---",
		"version": "19.07-SNAPSHOT",
		"revision": "r0+12242-aef001ae93",
		"target": "ipq807x/routing",
		"description": "--- 19.07-SNAPSHOT r0+12242-aef001ae93"
	}
}

It appears you are using firmware that is not from the official OpenWrt project.

When using forks/offshoots/vendor-specific builds that are "based on OpenWrt", there may be many differences compared to the official versions (hosted by OpenWrt.org). Some of these customizations may fundamentally change the way that OpenWrt works. You might need help from people with specific/specialized knowledge about the firmware you are using, so it is possible that advice you get here may not be useful.

You may find that the best options are:

  1. Install an official version of OpenWrt, if your device is supported (see https://firmware-selector.openwrt.org).
  2. Ask for help from the maintainer(s) or user community of the specific firmware that you are using.
  3. Provide the source code for the firmware so that users on this forum can understand how your firmware works (OpenWrt forum users are volunteers, so somebody might look at the code if they have time and are interested in your issue).

If you believe that this specific issue is common to generic/official OpenWrt and/or the maintainers of your build have indicated as such, please feel free to clarify.

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