I believe the ASA's were true L3 gateways, but they may not have been as picky about ARP, or basically may have just played nicely with each other. There are certainly other options as to why, but I wouldn't worry about trying to figure out why it was actually working. Move on and just fix it. *It would be a good idea to make sure the switches are indeed operating in L2 mode with just a management interface and are not doing any routing. For incoming services from both ISP's you would look under the "port forwarding" section of the firewall configuration page. Each of the options let you choose the up-link (Internet 1 or Internet 2), so you should be able to use both connections for different services inbound. You'll need to figure out if you which method you were/are doing for inbound connections. For true DMZ, you'll need to define in the firewall that the "DMZ VLAN" can't talk to the internal "Data VLAN" as the "default" rule. Then make individual rules to allow communication as necessary. Meraki's essentially have all VLANS as same security zone (ASA lets you set security levels so that DMZ can't talk to zone with a higher security level) These articles will be helpful: Port forwarding and NAT rules: https://documentation.meraki.com/MX-Z/NAT_and_Port_Forwarding/Port_Forwarding_and_NAT_Rules_on_the_MX Creating a DMZ on the MX https://documentation.meraki.com/MX-Z/Firewall_and_Traffic_Shaping/Creating_a_DMZ_with_the_MX_Security_Appliance T-800
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