Have you checked the documentation? The MX can be configured to use both of its uplinks for load balancing. When load balancing is enabled under Security & SD-WAN > Configure > SD-WAN & Traffic shaping, traffic flows will be distributed between the two uplinks. The load distribution is based on the WAN 1 and WAN 2 throughput configured under Uplink configuration, such that the uplink with more throughput will distribute more flows. In the example below, WAN 1 is configured to pass 50Mb/s, and WAN 2 is configured to pass 10Mb/s. Since the download speed ratio is 5/1, for every five flows sent over WAN 1, a single flow will be sent over WAN 2: https://documentation.meraki.com/MX/Firewall_and_Traffic_Shaping/MX_Load_Balancing_and_Flow_Preferences I suggest you to read the documentation.
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