I have a situation where I am implementing a cloud based SIP phone system with all my users behind and MX100 configured with dual WAN. I have the MX configured to send all traffic out WAN 1, but continue to see phone traffic traversing WAN 2. I have my traffic shaping policies set to give priority to all SIP traffic, and my question is, would my traffic shaping override the "send all traffic out WAN 1" statement, and look at the available bandwidth of both WANs combined? Or would it only look at the available bandwidth of WAN 1?
I have seen something similar happen when there is a momentary outage on the primary circuit. The MX keeps sending flows out the backup circuit until those flows end before moving them back. The problem is some flows are long lived and can take a very long time to failback.
I've found causing a failure on the back path (such as unplugging the backup circuit and plugging it back in again) forces everything back onto the primary circuit.