Is your switch a managed switch and does your router support VLANs? If not, this won't be possible. You would follow the instructions for your specific switch to associate the different ports on the switch to the different VLANs and you would create the VLANs on the router. If either your router or switch doesn't support VLANs, this won't work. The synology-connected ports on the switch should be PVID in each of the VLANs subsequently (port 1, VLAN 1, port 2, VLAN2, etc.). The switch port that connects to the Mac (and to the router) would be a trunk port for all those VLANs and you would create new virtual interfaces on the Mac for the 10GbE port for all those VLANs. What will happen is that you basically will have four different subnets in your network with your 10GbE port on your Mac connected to all 4 with 4 different IP addresses in the 4 subnets and the 4 ports on the synology each only being in one of the subnets. Make sure that your router blocks traffic between the subnets so all traffic will flow just through the switch staying in the respective subnets and won't go to the router. The SMB client on the Mac will recognize 4 1 gigabit interfaces on the server and will connect the 4 virtual interfaces it knows about on your Mac to them and you will get a full 4 gigabit bandwidth to the Synology. The problem is that all this setup is wildly different on the different brands of routers and switches, so not really possible to create a tutorial that would work everywhere.
Also had to set the MTU size to 1496 because of some weird issue. Here's the note from that:
In the end this turned out to be a problem with the Dell U4025QW's built in network switch (software/driver issue). I noticed I was seeing the symptoms described here:
https://www.snailbook.com/faq/mtu-mismatch.auto.html
Where my SSH connections to the NAS would lock up.
After experimenting a bit I found that setting a MTU on the Mac's VLAN virtual interface to 1496 instead of 1500 would fix the issue, both the SSH session locking up and multichannel SMB breaking Finder.
Activity monitor now reports ~350MB/s when copying files from my NAS.
u/Exotic-Grape8743 this might be the issue with your USB-C interface as well. Try setting a low MTU on the VLAN virtual interface (1400 e.g.) and work up from there.
All from https://www.reddit.com/r/synology/comments/166k8t2/comment/jyqq440/?utm_source=share&utm_medium=web3x&utm_name=web3xcss&utm_term=1&utm_content=share_button