An MX can work with either a public or private IP address on its WAN interface. The only requirement is that it can talk to the Meraki cloud on the Internet. You can copy your 5512 "outside" interface settings to the MX, even though it is a 10.x.x.x addressing. However you also need to specify some reachable DNS servers - otherwise the MX wont be able to do a DNS lookup to get to the cloud. You might have some specific ones you are allowed to use, or you might be able to use Google's, 8.8.8.8 and 8.8.4.4. And obviously you need to specify the default gateway which is the same as the default route on the ASA. If the MX is not coming up connect to the local status page and see what error it is reporting. https://documentation.meraki.com/zGeneral_Administration/Tools_and_Troubleshooting/Using_the_Cisco_Meraki_Device_Local_Status_Page#MX.2FZ1_Series If the ASA has a VLAN tag configured on its outside interface make sure you configure the same tag on the MX WAN interface. Does your Meraki MX plug directly into your default gateway? If it doesn't then you'll need who ever is in charge of the default gateway to do a "clear arp" on it. Otherwise the default gateway wont know that the MAC address for the assigned IP address has changed.
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