Mist can configure multiple WANs per traffic steering.

N/A

On the Mist side, it is as easy as creating a Traffic Steering profile with 2 or more WANs as next hop, the order (from top to bottom) of the WANs will determine who is primary and backup. Make sure to select "ordered".

On the SRX side, the mechanism that Mist installs is: each Traffic Steering profile represents an APRB routing table, each WAN also has its routing table, the APRB routing table from traffic steering will be populated using RIB rules with the default routes of each WAM routing table, the RIB has a policy which will install each WAN RT default route on the ABRP RT with a different preference, this preference will resemble the config of primary and backup seen on the Mist portal; the default route installed from the WAN configured as primary on Mist, will always have the lowest priorities, while the other routes will have higher and higher preferences based on the position in the Mist portal.

The failover mechanism works by each WAN having it's own RPM probes monitoring its respective interface link, if the primary WAN were to fail RPM monitor would fail, therefore IP-Monitoring would detect it and install a 100.99.255.2/32 route in the Primary WAN Routing Table, the RIB policy has configured a conditional which detects if such route exists in the Primary WAN Routing Table and if the condition is true then it stops send the the Primary WAN RT default route to the APBR routing table (the traffic steering), so the next default route (in preference) will take over as the "*" selected default route for the traffic steering.