Currently I am learning about load balance method available on SonicWALL. I have some question regarding the load balance methods for 1) round robin 2) spill over 3) percentage. Assuming there are two connection available for Internet connection:

  1. Does round robin utilize all link so the bandwidth throughput will be increased, for example if we had two 4 Mbps connection so the total throughput will be 8 Mbps?
  2. In spill over, does it also will increase bandwidth throughput if the primary link is overloaded?
  3. Percentage makes me a bit confused. If I have 2 links, can I use both links 100%? What is the downside if you assign 100% for both links?
  4. How do I chose the correct method, and can someone explain and give me an explanation on how to chose those method based on situation/advantage/disadvantages?
  5. Will all of those methods be shifted to the secondary link if the primary link fails?

1 Answer 1


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 route entries.
  • 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.

  • thank's for answer #Ron Maupin, but I already read the article/manual from sonic wall prior to asking in this forum... I need more explanation because I need it to explain to my boss in more depth
    – dchochan
    Commented Feb 10, 2016 at 2:56
  • I don't know what else to say about it. The text clearly details how each version works. What, specifically are you having trouble with?
    – Ron Maupin
    Commented Feb 10, 2016 at 2:57
  • Actually we want to implement it next month, but we confused whether to use active/passive or round robin... my boss asked me whether it possible to use all link so more bandwidth but the internet still working if one of ISP link down. So my choice would be round robin if it will utilize all of bandwidth available while prevent internet connection down if one of the link stop.
    – dchochan
    Commented Feb 10, 2016 at 3:22
  • That's what management always says until the first failure, then you will be tasked to change to Active/Passive. :)
    – Ron Maupin
    Commented Feb 10, 2016 at 3:24
  • I think next month, still wait the device bought from HQ in Japan. So what your opinion sir @RonMaupin should I use round robin or active/passive, my boss asked me if possible to use all link if possible because he do not want any waste(though in redundancy concept always cost double). Maybe we better chose spill over?
    – dchochan
    Commented Feb 10, 2016 at 3:31

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