Hello Abhishek.
There are advantages and disadvantages to creating a trunk between the Router and the Cisco 3850 switch. It all depends on what you want to achieve.
You would want to do this if:
- you want all of the routing of your network to take place at the Router
a) advantages of this include: to be able to apply security, access lists and other policies at a single location
b) disadvantages include single point of failure for routing - if you want to allow VLANs to span multiple areas of your network (if the Router is a layer 3 switch)
You would avoid doing this to:
- limit the extent of your VLANs to avoid broadcast traffic going to other access areas of your network
- Avoid a single point of failure. The 3850 switch is a layer 3 device and can do routing for all of the local VLANs.
Keep in mind that since you have an access port at the Cisco 3850 you will not create any VLANs beyond this device (lower down in your diagram).
To answer your other questions, please clarify:
What is the FW device and what is the purpose of implementing NAT at this location?
I hope this was helpful!
Laz