You can integrate other TrustSec capable devices into the environment, but it will be much more complex. And if I remember right, the Catalyst 3750 is not SGT capable. For your design you have to distinguish Trustsec classification which is done on the Network access device and Trustsec transport which is done in the rest of the infrastructure.
If you have a non-Meraki device for classification, then you need the Cisco ISE to do this job. If you have MR and or MS390 on the Edge, the Meraki native solution can do it. But after this classification for the end-device and/or end-user, the traffic has to be tagged and transported throughout the network. for that you need switches and devices that can process this additional field in the frame header. For me, Trustsec on the Meraki Platform is not the way to go as I do not want to have all the struggle with the MS390.
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