From the link:
Load Balance
Load Balancing can be utilized when both Internet ports are connected to two Internets connections and used at the same time. Using both the primary and secondary interfaces, load balancing enhances maximum bandwidth efficiency and reduces unnecessary waiting time. If one of the connections is lost, the remaining link will provide the network with Internet traffic until the other link is up again - similar to Smart Link Backup.
The Load Balance feature is ideal when the two Internet ports are connected to two different broadband Internet Service Provider (ISP) connections that have an equivalent bandwidth rating.
Intelligent Balancer
The Intelligent Balancer feature is automatically activated when Load Balance mode is selected. The Load Balancer will dispatch IP session packets based on the upstream bandwidth ratio of the two Internet connections. The upstream bandwidth monitoring is set to determine the ratio of outbound load balancing.
A special "round robin" algorithm is used to enhance the automatic Load Balance calculation. Round robin works on a rotating basis: when a server IP address is handed out, it moves to the back of the list. This step loops repeatedly as the next server IP address is received.
Bandwidth Ratio (weight equation)
The weight of equalization is calculated from the ratio of WAN1 upstream bandwidth and WAN2 upstream bandwidth. For example, if the rates are sets at 2M and 512K for WAN1 and WAN2, respectively, the ratio for the load balance will be 4:1. This means that sessions forwarded by WAN1 are four times the sessions of WAN2; but, if the load for WAN1 is over 60%, Intelligent Balancer will redirect the overflow to WAN2, avoiding traffic congestion.
Sessions
A session, defined by the Load Balance system, consists of these fields in a TCP/UDP packet:
Source IP Address
Destination IP Address
Type of Service (TOS)
Packets with the same session (that is, the same Source IP, Destination IP, and Type of Service) will be routed via the same interface. This avoids packets reaching their destination out of order, although port numbers are often used in conjunction with the Source and Destination IP addresses to determine the correct route of a packet.
If port numbers were used primarily, sessions could be directed through different interfaces, resulting in incorrect packet routing. Certain applications generate packets with several different port numbers, so both IP addresses and the Type of Service must be used for session identification.
For ICMP packets, sessions are not used, so each packet can be forwarded by either interface, packet by packet.
Dispatch Rules
In Intelligent Balancer mode, the Load Balance system will dispatch IP packets based on these rules:
The same sessions are forwarded (routed) to the same interface.
Different sessions are dispatched to different interfaces based on configured bandwidth ratios.
Sessions are dispatched based on a static bandwidth (upstream and downstream) utilization threshold of 60% with these routing sub-rules:
If the bandwidth utilizations of both WAN1 and WAN2 are under 60% (that is, both downstream and upstream utilizations are less than 60%), a new session will be routed based on rule (1) and (2) normally.
If the bandwidth utilizations of WAN1 or WAN 2 are over 60% (that is, either downstream or upstream utilizations are higher than 60%), a new session will be routed to the WAN with the higher available bandwidth.
If the bandwidth utilizations of WAN1 and WAN2 are over 60%, (that is, both downstream and upstream utilizations are higher than 60%), a new session will be routed to the WAN with the higher available bandwidth.
