It sounds like a compatibility issue between Meraki and SonicWall's LACP implementation. Have you checked if both sides (Meraki and SonicWall) have matching LACP settings, such as active/passive mode? Also, try verifying STP settings to ensure there’s no loop blocking the connection. ... View more

It looks like you’ve done a great job documenting everything so far! Since the affected ports are alternate uplinks, I’d start by checking STP settings, port roles, and any recent config changes. Have you noticed any looping or high latency issues? ... View more