SD-WAN

My understand with EdgeHA is, there shouldn't be asymmetric routing e.g. branch traffic egresses EC-1, return traffic should never ingress EC-2.

• Assuming a pair of ECs are deployed in EdgeHA
• MPLS-A terminates to EC-1, MPLS-B terminates to EC-2
• Local SVIs DHCP scopes etc. remain defined on the upstream layer-3 switches

We're seeing asymmetric from other sites to the location above.

What criteria do the ECs use to determine which is master/backup?

Would defining a bogus VRRP segment on the ECs be enough to designate EC-1 is the master or do we need to resort to route policies?

Thanks

In EdgeHA there is no Primary/secondary from an SD-WAN perspective. Tunnels are formed from both appliances over all underlays that you would normally attach to in a single box setup (MPLS-A and MPL-SB in your setup) to the remote locations. It doubles the number of tunnels you need. 

In your setup EC-1 will get access to the interface that connects to MPLS-B via the HA-cable and EC-2, and EC-2 will connect to MPLS-A vice versa. The EC that will do most of the forwarding ("primary" if you want to call it that) will be the one that is the VRRP-master as it attracts all outbound traffic. Inbound traffic arrives via the tunnel chosen by the remote side which is depending on various factors like tunnel affinity and precedence