NR7701 IP Passthrough: eth0 Down/Up events causing WAN disconnects on FRITZ!Box 7690 ML

Greg97

Hello,

I'm experiencing recurring WAN disconnects with a Zyxel NR7701 running in IP Passthrough mode connected to a FRITZ!Box 7690 ML.

The setup is:

- Zyxel NR7701 (ISP-provided firmware - A1 from Austria)
- IP Passthrough enabled - Mode Dynamic
- FRITZ!Box 7690 ML connected to the NR7701
- Ethernet link running at 2.5 Gbit/s Full Duplex

Several times per day the FRITZ!Box loses its WAN connection.

At the time of the outage, the NR7701 log consistently shows entries similar to:

esmd: Interface eth0, Status: Down
esmd: [ESMD]: unexpected empty / null payload
esmd: Interface eth0, Status: Up, Speed:2500, DuplexMode: Full

Immediately afterwards, DHCP activity appears in the NR7701 logs.

At the same time, the FRITZ!Box reports:

- Internet connection disconnected
- DHCPv4 no answer on DISCOVER

A few seconds later the connection is restored automatically.

What makes this interesting is that the cellular connection itself does not appear to be reconnecting. The NR7701 does not reboot, and the connection uptime to the provider / cellular tower is not lost. The issue seems to be limited to the Ethernet/IP Passthrough side, where eth0 briefly goes down and then comes back up.

Has anyone seen similar eth0 Down/Up events on the NR7701?

Could this be:
- a known IP Passthrough issue,
- a firmware bug,
- a 2.5G Ethernet negotiation problem,
- or something related to DHCP handling on the NR7701?

Any feedback or similar experiences would be greatly appreciated.

Thanks!

Additional information:

- The ISP has already investigated the issue.
- 5G has been disabled for testing purposes, but the problem still occurs while running on LTE only.
- The issue persists across multiple disconnect events.
- The FRITZ!Box is connected via IP Passthrough and obtains its WAN address via DHCP from the NR7701.