Something to keep in mind is the way failover functions with the USB port. We have used this in the past and found that the ISP link will be up but not passing traffic and this would prevent the MX from failing over to the USB or cause flapping. We have had very good success with the SA2100 instead of the DS as the SA2100 has an Ethernet port which let's the failover/load balancing work the way most people would expect. Just my two cents.
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