I am experiencing an issue with my network setup involving a Teltonika RUTX12 router and a Grandstream managed switch.
Network Topology:
Starlink (Bypass Mode) → WAN port of Teltonika RUTX12 → LAN port of RUTX12 → Grandstream Switch → Client Devices
The Starlink router is configured in Bypass Mode, so it is not performing any routing or DHCP functions. The Teltonika RUTX12 is acting as the main router and DHCP server for the LAN network.
Problem Description:
When the LAN port of the RUTX12 is connected to the Grandstream switch:
Client devices fail to obtain IP addresses.
Internet connectivity stops.
The network becomes unstable.
However, when I disconnect the LAN cable between the RUTX12 and the switch:
The RUTX12 immediately resumes assigning IP addresses correctly.
Internet connectivity works normally for directly connected devices.
This behavior is consistent and reproducible.
Important Details:
There is NO network loop.
There is no secondary connection between the switch and Starlink.
The switch is only connected to the RUTX12 LAN port.
Starlink is confirmed to be in Bypass Mode.
The RUTX12 WAN interface is configured as a DHCP client.
The RUTX12 LAN interface is configured with DHCP server enabled.
The WAN port is not intentionally bridged with LAN (as far as I can see).
Only one DHCP server should be active (on the RUTX12).
When the issue occurs, the router does not completely power off, but client devices cannot obtain IP addresses from the LAN.
Troubleshooting Already Attempted:
Confirmed Starlink is in Bypass Mode.
Checked for physical loops — none exist.
Verified correct cabling (Starlink to WAN, Switch to LAN).
Rebooted all devices.
Tested with different Ethernet cables.
Confirmed DHCP is enabled on the RUTX12 LAN.
Ensured no secondary router is connected.
Despite this, the issue persists whenever the LAN is connected to the switch.
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