That depends on the WAN circuits you are using. Really the only requirement is for each WAN port that you are using on the hub to be able to access the Meraki registry (essentially the internet) - so if you have direct internet connections then you will be fine, or if you're using an MPLS network that provides NATed internet access then you will be fine, for both these or a combination of them you should be able to run the MX in routed mode. Generally one arm concentrator is only a necessity if you want to do dynamic routing, have multiple DCs providing access to the same subnets, or if you need to provide the internet access from an MPLS network - although that's not a prescriptive list.
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