[M4300] M4300-24X24F (XSM4348S) Stacking, new environment, other brand switches

Now ofcourse the current servers and current "core" switch will disappear so:

• can I connect those other switches to the stack somehow? (2 of them are in hard to reach places..., and it will save me 7000 euros also if I choose for additional M4300 switches...)
  • You can connect them to stack switches but as just another uplink switch.  You need to setup a trunk link (basically port which tags all vlans) on both M4300 and existing switch in order to transit the VLAN across.  You can also connect it to both stacked switches via 2 cables if you want but for that you will need to create a LAG on both sides and make sure they use the same settings and then configure the LAG to be trunk or tagging all VLAN on both sides.
• can I still use VLAN's? I read something that VLAN's cannot be used with a Leaf-Spine setup? Or am I misunderstanding things?
  • VLAN's is not limited by network design topology.  As such you can use VLAN for leaf and spine.  You are definety miss understanding leaf and spine.   On M4300 you can create a Leaf and spine setup using stacking technology to build a single 8 port stack of switches (2 acting as Core and rest acting as distribution or end nodes)  or you can use the old method which is 2 stacked spine and each switch is hooked up to spine via 2 cables (1 to each switch) but only 1 is is forwarding state but other is in BLK state by spanning tree to prevent loops.  VLAN can be designed on the switching network and have nothing to do with topology or design implementation.
• regarding Windows Server 2016 I don't need to set up any distributed LACP, thanks to the build-in NIC teaming Windows has, so no issue here
  • Windows by default teams using Switch independent method (Not using LAG or LACP method) as such it will use active load balancing method (1 sided) to achieve teaming.  This is very limited in capabilty compared to true team LAG using layer 3 hash where load is more equally distributed but that is bit harder to setup as you need to have switch support.  M4300 should be able to provide both LACP and IP hashing methods to do any type of teaming to the server.

As you where to start.  I think first step for you should be document the existing network.  Build the network topology map and export the configuration.  Second step would be create action plan on whether you want to migrate the network offline or online.   The online migration is harder as you are literly building a network in parallel in order to transfer equipment to new stuff without much downtime and will require quite a lot of planning.  The offline migration is easier as long as you plan ahead the configuration and maybe even validate part of before taking the existing network offline.