A quick Internet search turns up Network > WAN Failover & LB directly from the source:
You need to choose a load balancing method. By default, the SonicWALL
will select Basic Active/Passive Failover as the method, but there are
four load balancing methods available:
- Basic Active/Passive Failover: When this setting is selected, the SonicWALL security appliance only sends traffic through the Secondary
WAN interface if the Primary WAN interface has been marked inactive.
The SonicWALL security appliance is set to use this as the default
load balancing method. If the Primary WAN fails, then the SonicWALL
security appliance reverts to this method instead of the ones
described below. This mode will automatically return back to using
the Primary WAN interface once it has been restored (Preempt and
failback to Primary WAN when possible setting) preempt mode). This
item has an associated Preempt and fail back to Primary WAN when
possible checkbox. When this checkbox is selected, the SonicWALL
security appliance switches back to sending its traffic across the
Primary WAN interface when it resumes responding to the SonicWALL
security appliance’s checks (the WAN’s physical link is restored, or
the logical probe targets on the WAN port resume responding).
- Per Connection Round-Robin: When this setting is selected, the SonicWALL security appliance load-balances outgoing traffic on a
per-destination basis. This is a simple load balancing method and,
though not very granular, allows you to utilize both links in a basic
fashion (instead of the method above, which does not utilize the
capability of the Secondary WAN until the Primary WAN has failed).
The SonicWALL security appliance needs to examine outbound flows for
uniqueness in source IP and destination IP and make the determination
as to which interface to send the traffic out of and accept it back
on. Please note this feature will be overridden by specific static
- Spillover-Based: When this settings is selected, the user can specify when the SonicWALL security appliance starts sending traffic
through the Secondary WAN interface. This method allows you to control
when and if the Secondary interface is used. This method is used if
you do not want outbound traffic sent across the Secondary WAN unless
the Primary WAN is overloaded. The SonicWALL security appliance has a
non-Management Interface exposed hold timer set to 20 seconds – if the
sustained outbound traffic across the Primary WAN interface exceeds
the administrator defined Kbps, then the SonicWALL security appliance
spills outbound traffic to the Secondary WAN interface (on a
per-destination basis). The user entry box should not have a default
entry and be left empty for the user. Please note this feature will be
overridden by specific static route entries.
- Percentage-Based: When this setting is selected, you can specify the percentages of traffic sent through the Primary WAN and
Secondary WAN interfaces. This method allows you to actively
utilize both Primary and Secondary WAN interfaces. Only one entry
box is required (percentage for Primary WAN), as the SonicWALL will
auto-populate a non-user-editable entry box with the remaining
percentage assigned to the Secondary WAN interface. Please note
this feature will be overridden by specific static route entries.
- Use Source and Destination IP Addresses Binding: This checkbox enables you to maintain a consistent mapping of traffic flows with a
single outbound WAN interface. This method uses only the source IP
address and the destination IP address when establishing
load-balancing assignments. The load-balancing nodes discards all
other information, including source or destination TCP port numbers.
The SonicWALL assigns interfaces based on individual traffic flows (source IP address and TCP port to a destination IP address and TCP port). A single traffic flow will not be spread across multiple links. This prevents the problems caused by out-of-order packet delivery (slows TCP and kills many UDP applications, especially real-time traffic).
Round-robin - A single traffic flow needing more bandwidth than a single link will still be capped at the bandwidth of the link to which it is assigned. With a lot of traffic flows, you will get close to the combined bandwidth of both links since different traffic flows will be assigned to different links.
Spillover-based - You specify at what percentage of the bandwidth the primary link uses before starting to use the secondary link, but, again, each traffic flow will only flow over a single link, so a traffic flow needing more than the configured percentage will not use the second link and be capped at the bandwidth of the primary link.
Percentage-based -. You can define the the flow percentages each link gets. For instance primary 75%, secondary 25% will use the primary link for the first three flows, but the fourth flow (assuming the first three flows are still active) will be assigned to the secondary link. This is useful if the different links have disparate bandwidths.
You should consider that the primary use for multiple WAN connections is failover. If you use bandwidth larger than one link, you will be crippled to some degree in the event of a link failure. This may or may not matter to you, and management often says this doesn't matter until a failure actually happens.