1) Make sure you connect them correctly. So port 1 of active goes to port 3 of switch 1 and port 2 of active goes to port 3 of switch 2. Then port 1 of spare firewall goes to port 4 of switch 1 and port 2 of spare firewall goes to port 4 of switch 2. 2) Make sure the Sonicwall uses 802.3ad (or AX) which is LACP negotiation of the channel to bundle them. Meraki switches will not form bundles statically. If you are unsure, connect any one of the firewalls to 1 of the switch ports and run a dashboard capture on it and use the capture filter: ether host 01:80:C2:00:00:02 and you will definitely see messages coming from the MS switch but you should check if there are messages coming from the Sonicwall. If not you have a static aggregate and need to reconfigure that.
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