Help! Firewall interface / VLAN / VoIP phone configuration / Messy environment

Hello everyone,

I'm looking for assistance with some messy network configuration problems I've been having (I'm a new-ish Sys Admin at an MSP - haven't done a whole lot of networking in larger environments). The environment has pretty much been held together with duct tape, it seems, and I'm here trying to clean up the mess.

https://imgur.com/MpFghRT Here's a brief overview of the relevant parts of the environment (they do have ESXi hosts, a DC, file server, app servers etc etc, but I don't think they're relevant for now)

Basically, I'm trying to setup VLANs for their VoIP phones so the phones in their secondary building (going through the AirFibers) can communicate with the phones in their primary building, and so they're also segmented for QoS purposes.

I attempted to create a L2 Bridge between the X0 and X3 LAN interfaces on the SonicWall, but quickly discovered that DHCP cannot be done over a L2 bridge (the SonicWall is currently doing DHCP for their entire network).

Switching DHCP over to their domain controller is an option that I've considered, but have not implemented yet, as I'm not entirely sure if this is the best solution (or even a solution at all).

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HOWEVER, they are now going to be switching over from the SonicWall to a Sophos XG appliance.

It seems you can tag VLANs to a bridged interface of a Sophos XG using the CLI, but I've been reading mixed answers on whether or not this is truly doable, and also whether or not the Sophos can do DHCP on bridged interfaces (I'm fairly sure it can).

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Should I use their DCs for DHCP? Do I need to use a switch for interVLAN routing? Should I reconfigure the entire switch layout and do away with the dual LAN interfaces?

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I'm probably leaving out important details (let me know if you need any more information!), but I'll leave it at this for now. Also, I haven't done a whole lot of network diagramming, so I apologize for the sloppy one I put together.

Any direction on where I should focus my efforts would be VERY much appreciated.

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Thanks :)