Linksys WRT1900ACS local LAN stops working, wlan and WAN still work

"Yesterday it still worked"
Oh if I had a penny for every time I heard that from a customer...

and now: it happens to me.
Yesterday, my network was working fine, today the Ethernet behaves badly:

  • after reboot, all works fine for a while. Sometimes 30s, sometimes 8 minutes.
  • after some time, all connectivity to LAN ports seems to stop
    -- including DHCP: computer on the LAN get no config
    -- including ICMP
    But when this problem hits, the WAN port and WLAN still work fine. I am actually using that route at this very moment to type this...
    The front panel LEDs indicate traffic on at least one LAN port, but the device connected there (Raspbi) can no longer communicate on the network. Neither can any device on a secondary switch connected to the router.

KI recently updated from 19.x to the currently running
OpenWrt 21.02.3 r16554-1d4dea6d4f / LuCI openwrt-21.02 branch git-22.119.37126-a993714

I am familiar with Liunux, but a bit unfamiliar with the debug toolset provided by OpenWRT.

Why would the switch indicate traffic (LED) but not pass any traffic. Actually: I should probably verify on the raspi if there is traffic on the Ether...

I am a bit confused by interface names such as lan1@eth0 shown by ip address.
ifconfig seems to show these as lan 1 to lan4 respectively and also shows an eth0.
ifconfig lan3 down shuts down the interface to the raspi, ifconfig lan3 up brings it back up - still: no communications.
Also: the TX packet counter on lan3 counts up, RX stays constant.

any tips how to debug - or even better: fix this?

One more thing: when I query the state with ip address, the LOWERLAYERDOWN./.UP state tracks the state of the ethernet port correctly - yet: no traffic on any LAN port, but wlan and wan working quite well.

(h)top indicate pegged device?

lanx@ethx indicates cpu switch port lanx is utilising; currently only 1(of 2) in play.

htop was clear, hardly any load or traffic on the machine

Seems I fixed the problem: I reset the config to default and manually rebuilt it: I did not dare to import the backup, in case the problem is in the config.
The router seems to work fine now, including the LAN ports.