Ubiquiti USG stopped routing through WAN1

Since a few weeks, I have a strange problem with the Ubiquity USG. The USG no longer routes Internet traffic from het internal network to the Internet via the WAN port, but it does so via the WAN2 port.

First a drawing of the relevant parts of the network:

The internal network is the 192.168.178.0/24 and it is connected to the USG LAN port. Previously, the USG WAN1 port was connected to the connection network 192.168.17.0/24. The IP address and default gateway on the WAN side was obtained via DHCP from the Ziggo modem. The Ziggo modem then connects to the Internet. Both the USG and the Ziggo modem do NAT. This has worked for over a year, with reboots, upgrades and temporary power failures.

Suddenly, the Internet became unreachable from the internal network. What worked was: – From the laptop, I could access the status-page of the Ziggo-modem – From the CLI of the USG, I could ping 8.8.8.8 – There was nothing in the logs

When connected to WAN1, I have the following routing table:

vbash-4.1# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 192.168.17.1 0.0.0.0 UG 0 0 0 eth0
10.128.0.0 192.168.178.15 255.255.255.0 UG 0 0 0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
192.168.17.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
192.168.100.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1.101
192.168.102.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1.102
192.168.178.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1

When connected to WAN2, I have the following:

vbash-4.1# netstat -rn
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
0.0.0.0 192.168.17.1 0.0.0.0 UG 0 0 0 eth2
10.128.0.0 192.168.178.15 255.255.255.0 UG 0 0 0 eth1
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
192.168.17.0 0.0.0.0 255.255.255.0 U 0 0 0 eth2
192.168.100.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1.101
192.168.102.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1.102
192.168.178.0 0.0.0.0 255.255.255.0 U 0 0 0 eth1

I did all the usual thinks: reboot of both the USG and the Ziggo modem, re-provisioning the USG, switching the USG to a static IP on the 192.168.17.0/24 network.

Then I connected the 192.168.17.0/24 network to the WAN2-port. This port has been specially configured as WAN port, so it is not a LAN. And the Internet became accessible again. In the logging, there were messages of a network failover which takes 20 seconds. The WAN2-port is configured as DHCP too.

When 192.168.17.0/24 is connected to WAN1 and after a reboot of the USG, I can access the Internet from the laptop about 10-15 seconds, before it shuts down again.

The WAN1 port is not broken, otherwise, I would not be able to access the Ziggo status page. The USG should know where the Internet is, otherwise I would not be able to ping 8.8.8.8 from the USG CLI. The USG does routing, otherwise it would not work with the WAN2 port. I can find nothing in the logs of the USG.

I do not know where to look or how to continue my debugging. Does anyone know what the problem might be (and how to solve it)?

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