If the root of your network is a Cisco device running RPVST, and the Meraki devices are at the edge of the spanning-tree then there is a fair chance you'll get it working without having to implement MST, or anything else special - the Cisco switches transmit a standards based BPDU as well as the Cisco proprietary BPDUs. Any issues mainly occur if a Cisco device isn't the spanning-tree root, or if you're expecting to 'hop' across a non-Cisco device, e.g. Cisco <-> Meraki <-> Cisco. If your Cisco switches are root then you need to drop your priority on them to closer to 0, e.g. 8192, 4096, or even 0 itself. As default the Meraki devices use a default STP priority of 32768 (which I believe is what Cisco use), so if you have your root set at 61440 then its really unlikely it will actually ever be your root.
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