Since you are replacing an ASA with an MX, I'll just have to say this. A Meraki MX100 is not a Cisco ASA firewall. So don't be surprised if there is some functionality in an ASA that isn't present on an MX. I've had my fair share of rogue Sales persons, thinking an MX can simply replace an ASA, without having looked into the running configuration. You can not assign an IP address to an physical interface of an MX (or any other Meraki product). So in order to obtain what you are depicting, you'll have to create a VLAN on the MX, and assign it an IP, and a SVI interface on your core switch. Just as others already have mentioned. Basically, what you are doing is, a Layer 3 topology as described in https://documentation.meraki.com/Architectures_and_Best_Practices/MX_and_MS_Basic_Recommended_Layer_3_Topology Assuming you are handling DHCP somewhere else, then creating the VLAN on the MX, remember to disable the DHCP server, that is created by default.
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