Let’s suppose the Routing Information Base (RIB) of the firewall has multiple equal-cost paths to a single destination. The maximum number of equal-cost paths defaults to 2. ECMP chooses the best two equal-cost paths from the RIB to copy to the Forwarding Information Base (FIB). ECMP then determines, based on the load-balancing method, which of the two paths in the FIB that the firewall will use for the destination during this session.
ECMP load balancing is done at the session level, not at the packet level—the start of a new session is when the firewall (ECMP) chooses an equal-cost path. The equal-cost paths to a single destination are considered ECMP path members or ECMP group members. ECMP determines which one of the multiple paths to a destination in the FIB to use for an ECMP flow, based on which load-balancing algorithm you set. A virtual router can use only one load-balancing algorithm.