For the localized application visibility features (DPI and NBAR2) to be able to classify most application traffic, it is important that the WAN Edge router sees network traffic in both directions.
In dual-WAN Edge sites without any policy enabled, equal cost paths exist over each transport and to each WAN Edge router, and network traffic is hashed depending on fields in the IP header.
Traffic is unlikely to always be forwarded to the same WAN Edge router in both the LAN-to-WAN direction and the WAN-to-LAN direction. To maintain symmetric traffic, it is recommended to set up routing so that traffic prefers one WAN Edge over another at dual-WAN Edge router sites.