My question was if it was possible for the U3421WE to deliver Ethernet traffic only to device connected with USB-C?* No. Can you please point me to where this is mentioned in the manual?* It is not stated in such a black and white way. It simply states that the RJ45 Ethernet port supports 10/100/1000. The presumption by the manufacturer was that all users would know from that statement that Ethernet follows the chosen PC via video in port.
My setup looks like this: [Work laptop] --USB-C--> [U3421WE] <--DP+USB-B-- [Thunderbolt3 Dock] <--USB-C-- [Personal Laptop] Usually I'm running both laptops in a PBP setup and have a "Switch USB" keyboard shortcut that allows me to change which laptop I am controlling. When switching USB, apart from the keyboard and mouse, the Ethernet is passed to the other laptop as well (which makes sense as I'm assuming the monitor is acting as a Dock). The problem is that when switching USB, the network connection drops for a few seconds until the first laptop establishes a new connection via WiFi. I'm often connected to a VPN on my work laptop, and the connection drop makes me lose my VPN connection (which causes other issues). Is there a way to make the Dell U3421WE deliver Ethernet only through the USB-C cable in such a way that "Switch USB"  does not pass control of the Ethernet to the other laptop?
U3421WE, make the Ethernet work exclusively with USB-C?