The DHCP configuration DNS settings in Meraki tells each client making a DHCP request which DNS servers to use. If you specify multiple DNS servers, the client will make its requests based on its own algorithm. For Windows this is detailed here. It can be quite complicated. As I understand it, the MX devices don't have DNS servers - no DNS caching. HTTP caching is for caching of HTTP content only (e.g. web pages, graphics etc.).
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