Hi, I have a problem at work with network collapses when connecting a router to a VLAN. Let me explain:

We have a simple managed switch (TP-Link TL-SG1024DE) divided into three VLANs using 802.1Q:

• VLAN 2: Ports 1-6
• VLAN 10: Ports 9-16
• VLAN 30: Ports 17-24

Each VLAN has its own router connected in untagged mode with its PVID correctly configured.

The problem: When connecting a cable from a hospital's internal network to VLAN 2, the switch collapses completely. It works for a moment after rebooting and then goes down again. Interestingly, VLAN 30 is almost always the one that drops, not VLAN 2 where the problematic cable is connected.

What we have checked:

• Disconnecting the cable from VLAN 2 restores everything to normal
• Loop Prevention is enabled but the switch provides no information about which port is generating the loop
• The internal network is completely undocumented, spread across multiple floors with a massive amount of cabling

Questions:

• How can we identify and isolate the loop without knowing the internal network topology?
• Is there any way to make the connection safe from the switch side without it collapsing?

We are aware that having a proper network topology would help enormously, but since it is an undocumented legacy network that option is not viable.

P.S. If you know of any switch with more advanced VLAN management and loop control options, recommendations are very welcome.

Thanks.