As noted in section 6.6, a UCS can be designated as Backup. When there is at least one non-Backup UCS available at an SD-WAN Edge (availability of a UCS is described by [R22] and the explanatory text around it), Ingress IP Packets are not forwarded toward the egress UNI over a UCS that is designated as Backup. However, since Backup UCSs may have lower bandwidth and/or higher cost, it may be desirable to restrict which Application Flows are permitted to use them. This control can be achieved using the BACKUP Policy Criterion. It can have the value Yes or No.
[R32] Application Flows MUST NOT be forwarded toward the destination egress UNI over a UCS that is designated as Backup if a non-Backup UCS to the destination egress UNI is available.
[R33] If the Policy Criterion BACKUP=No is assigned to an Application Flow, then the Application Flow MUST be discarded if only Backup UCSs to the destination egress UNI are available.
Note that Application Flows that have BACKUP=yes will likely be more resilient than those with BACKUP=no.