First step is to power down the DS412+ and anything else in your network that needs re-cabling. Make sure to leave at least 2 ports free for the DS412+. Find the two Gig-E ports on the back of the DS412+ and hook it up to any two ports on the Smart Switch. In my case I made the Netgear GS108T the main switch and hooked it up to my Wireless router that connects to the Internet. A second Netgear switch is hooked up by a long cable to another room where the desktop and Xbox reside. Across the living room to the TV I have my old Linux NAS hooked up. Power everything up and make sure everything is communicating as is.
Configure Link Aggregation on GS108T
The GS108T comes with Windows software called SmartControlManager. Alternatively you can access it via a Web Browser if you know its IP address. How do you find it's address aside from using Smart Control Manager? My LAN uses the Wireless router as a DHCP server. So I logged on to that device and listed all DHCP client. Using the names I found the GS108Tv2. A quick browser launch and an http://
From here I choose the Switching tab which immediately shows me the active ports. Follow LAG -> Basic -> LAG Configuration. Select the LAG1 entry (since this is an 8-port switch, you can have a maximum of 4 LAG). This is where it confused me at first. Selecting LAG1 populates the entry fields above it. Enter a Description, I chose "diskstation" then LAG Type of "LACP". Hit Apply.
Move to the LAG Membership option. From here select LAG1 then choose the ports the DS412+ was hooked up to. There is a yellow button that is equivalent of Select All. This works only if you have one device running LAG on this switch. Hit Apply.
That's it for the GS108T. Now to enable it on the NAS.
Configure Link Aggregation on Synology DS412+
Login to DSM on the DS412+ then launch Control Panel -> Network -> Network Interface -> Create