Monday, September 20, 2021

Puzzling multi-network routing behavior

Hello, I have a puzzle. I have sought a solution through the usual Googling, and even found a Spiceworks thread from someone with almost the exact same puzzle! But alas there was no resolution there, and it was posted in 2015.

Cast of characters:

- A new virtual hosting system to be configured in the lab, connected to an isolated layer 2 switch. It is destined for an industrial plant site, and will be set up in the lab using the subnet that belongs to that site. This subnet is 172.20.149.0/24.

- A laptop sitting in the lab. Its ethernet adapter is connected to the aforementioned layer 2 switch, with a static IP of 172.20.149.11 (no default gateway obviously). Its WiFi adapter is connected to the corporate WiFi, and has a DHCP address of 172.24.201.27 and an auto-configured default gateway and DNS etc. This is also a /24 subnet.

- My colleague, who is trying to set up the new virtual cluster remotely, as we're all still WFH. She uses our secure remote access system to get a screen session with the laptop via the WiFi adapter.

The mystery is thus:

So far there is nothing on the local private 172.20.149.0/24 subnet configured aside from the laptop. But when she probes for certain addresses, she gets answers! From actual devices that are at the actual plant site. I would have thought this to be impossible. The route table is clear- the interface for destinations in 172.20.149.0/24 is 172.20.149.11

But when I ping 172.20.149.1, instead of getting "Destination host unreachable" every time (it did it once, then never again), Windows for some reason decides to try the WiFi adapter, which of course leads to the corporate WAN and eventually to the plant. I verified with tracert.

How can I convince Windows to be LESS helpful and to just give up if it finds nothing on the local subnet? I do not want it to try the other adapters. I feel like there must be a setting for this, but so far have utterly failed to find it.



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