After much troubleshooting, I discovered an issue when using the WD15 docking station with a Dell Precision 5510 laptop if the Cisco Systems VPN Client 64-bit software is installed and especially if Symantec Endpoint Protection exists on the laptop together with Cisco VPN (Symantec Endpoint Protection can manage the enabling and disabling of a wired ethernet connection).
To give some background, I provide I.T. support for a company that has deployed Dell laptops to its staff for many years. In our environment, our Dell laptops run Windows 7 Pro 64-bit. In this environment in the past, we've never had any issues using the Cisco Systems VPN Client with any Dell laptop that either has a built-in ethernet port or a USB ethernet dongle (including the Dell Precision 5510 when used with its included USB-C ethernet dongle). But I've discovered ethernet connectivity problems from the WD15's wired ethernet connection when used with a Dell 5510 when Cisco VPN is installed (regardless of whether or not a VPN connection is active).
With the WD15 connected to the 5510 and with a wired network connection attached to the WD15, if Cisco VPN is installed and if Symantec Endpoint Protection is not installed, when after starting up Windows 7 and then logging into Windows, sometimes within the first few minutes, the wired ethernet connection will then drop connection (an exclamation mark will appear over the wired network icon on the taskbar of Windows). If the WD15 is then disconnected from the 5510 and then reconnected to it, wired network connectivity will be restored, and it won't lose connectivity throughout that session of Windows.
If Cisco VPN and Symantec Endpoint Protection are both installed on the 5510, more problems occur. The same issues of losing wired network connectivity will occur within this setup. Unplugging the WD15 from the 5510 and then plugging it back in will restore wired network connectivity, but within minutes, wired network connectivity will be lost once again. Every time that the WD15 is disconnected and reconnected, this cycle will continue to occur (restored wired network connectivity, then lost wired connectivity within minutes). Furthermore, when wired connectivity is lost, if the WD15 is left connected to the 5510, then within minutes, other issues can occur. Functionality of devices connected to the USB ports of the WD15 will then begin to no longer function. Also, attempting to put the 5510 into sleep mode can result in long delays until it goes to sleep (as long as the WD15 is still connected to the 5510). Again, disconnecting and reconnecting the WD15 will fix all of this, but the problems will cycle once again when wired network connectivity is lost.
I've tried using two different versions of Cisco Systems VPN Client 64-bit (5.0.07.0290 and 5.0.07.0440), and the same problems occur regardless of which version of Cisco VPN is installed. I've also tried 5510 BIOS updates, Thunderbolt firmware updates, Thunderbolt controller driver updates, and other WD15-related driver updates including the Realtek USB GBE ethernet controller drivers, Intel HD graphics drivers and RealTek USB audio drivers. After all of these updates, the same exact problems occur as long as Cisco VPN is installed.
FYI, the Cisco VPN software includes a component in the "Networks" section of the Device Manager called "Cisco Systems VPN Adapter for 64-bit Windows." This component interacts with the wired ethernet drivers and wifi drivers.
After all the testing and troubleshooting, I can only conclude that the WD15 has a problem when Cisco VPN is installed. Unfortunately, this is a critical issue for us because we've been deploying Dell Precision 5510s, and anyone that uses the WD15 with the 5510 will run into the problems I've described.
If anyone from Dell is monitoring this, if there's any chance that you can look into this issue and find a resolution to this, it would be greatly appreciated. Again, this issue has never come up with any other Dell laptop over the years that we've deployed (regardless of whether the laptop has its own built-in ethernet port, whether it uses a USB ethernet dongle, or whether the laptop uses an ethernet port on another docking station such as the Dell D3000 SuperSpeed USB 3.0 docking station or the Dell E-Port Replicator PRO2X docking station).