When an active/active HA peer fails, its sessions transfer
to the new active-primary firewall, which tries to use the same
egress interface that the failed firewall was using. If the firewall
finds that interface among the ECMP paths, the transferred
sessions will take the same egress interface and path. This behavior
occurs regardless of the ECMP algorithm in use; using the same interface is
Only if no ECMP path matches the original egress interface will
the active-primary firewall select a new ECMP path.
If you did not configure the same interfaces on the active/active
peers, upon failover the active-primary firewall selects the next
best path from the FIB table. Consequently, the existing sessions
might not be distributed according to the ECMP algorithm.