WAN link load balancing

WAN link load balancing

In the same way that incoming traffic can be load balanced, outgoing or WAN traffic can also be load balanced and for the same three reasons.

1. Reduce the places in the work flow where a single point of failure can bring the process to a halt.

2. Expand the capacity of the resources to handle the required workload.

3. Have it configured so that the process of balancing the workload is automatic.

Often, it can be just as important for an organizations members to be able to access the Internet as it is for the denizens of the Internet to access the Web facing resources.

There is now a WAN Load Balancing feature located in the Network section of the GUI (“WAN LLB”).

As part of the new WAN Load Balancing feature, the FortiOS 5.2 Router > Static > Settings GUI page has been removed. WAN Load Balancing should be used instead of the 5.2 ECMP Load Balancing Method settings. The 5.2 Link Health Monitor definitions are now only available from the CLI.

WAN links

The basis for the configuration of the virtual WAN link are the interfaces that comprise it. As interfaces are added to the “wan-load-balance” interface, they are added into the calculations that comprise the various algorithms used to do the load balancing.

  • While most of the load balancing algorithms are based on equal distribution or weighted distribution, spill over does rely on which interface is first in the sequence, so this should be kept in mind when adding the interfaces.
  • The interfaces in the virtual WAN link can be disabled if necessary if work needs to be done on an interface without interfering with the performance of the link.
  • There is no requirement that the interfaces be those labeled on the hardware as WAN interfaces.
  • In the GUI, to help analysis the effectiveness of the algorithm being used and its configuration, there is a graphic representation of the bandwidth usage of the link.

Load balancing algorithm

Once the interfaces involved has been configured the next step is to determine how the workload will be distributed. 5 load balancing algorithms are available to choose from.


This is a very straight forward method of distributing the work load based on the amount of packets going through the interfaces. An integer value assigns a weight to each interface. These weights are used to calculate a percentage of the total bandwidth that is directed to the interface.


  • There are 2 interfaces
  • Interface #1 is assigned a weight of 5 because it is a 5 MB connection. (There is no requirement to match the weight to the capacity of the connection. It is just a simple way of optimizing the differing capacities in this case.)
  • Interface #2 is assigned a weight of 3 because it is a 3 MB connection.
  • The total weight is 8 so interface #1 gets 5/8 (63%) and interface #2 gets 3/8 (38%) of the traffic.

Wan Link Load Balancing Algorithm


The session algorithm is similar to the bandwidth algorithm in that it also uses an integer value to assign a weight to each interface. The difference is that the number of sessions connected is what is being measured and not the packets flowing through the interfaces.

WAN Link Load Balancing Spillover


This entry was posted in FortiOS 5.4 Handbook and tagged , , on by .

About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

29 thoughts on “WAN link load balancing

  1. Sharjil

    Facing issue with WAN LLB , 3 ILL links are combined to Virtual WAN interface & algorithm is “Source IP” based, but only 1 link gets choked & rest 2 are only consumed only around 30-40%.

    1. Mike Post author

      I prefer to use bandwidth on mine for LLB. Using source it is possible that you coincidentally just have all of the big hitters going through the same pipe.

  2. Bob

    i am having issue with mine (2 isp), when i connect to other fortigate outside the office i get disconnected because it uses the 2nd internet connection and the destination fortigate disconnects me.
    It seems to happend with other things also.

    1. Mike Post author

      In situations where you use WLLB and you are dealing with something that is smart enough to know that your IP (or the source in general) is switching up it will be beneficial to use routes. For instance, when I have WLLB enabled I set static /32 routes for the destinations that I know need to stay on a single connection (like a FortiGate I’m trying to administer remotely etc) to go out a specific pipe.

      Otherwise, you will constantly get booted out of the device you are trying to manage which is quite annoying.

  3. Bob

    i havent tested/read about it yet, but lets say you have a policy route that uses wan1 for X reason, will it use wan2 in case the wan1 is down?

    1. Mike Post author

      You would make two static routes (regular static routes like you do your default gateway routes….no policy route) that have the same destination (the remote device or service that requires the connection to stay on one WAN connection and not bounce between the two) the routes will be identical except the destination interface and gateway address will be different. The one you wish to be your primary you just make it have a lower administrative distance so that it takes precedence and if that connection fails the other route will take over.

    1. John Terry

      Could you please explain me more details on (regular static routes like you do your default gateway routes….no policy route) that have the same destination (the remote device or service that requires the connection to stay on one WAN connection and not bounce between the two)? I would like to have user connection stay on only WAN link because now user internet connection is being kick out because of two difference IP WAN interface.

      1. Mike Post author


        If you have a specific user that you would like to stay on a specific WAN connection then policy routes tend to be the best). If that user is using only a specific resource that you can tie to an IP Subnet or destination then you can make a static route specific for that destination. This, of course, would force all people to use that pipe though. Policy routes can be very powerful when done properly.

  4. Junior


    In the caso of two ISP with different bandwidth, for example 5 and 3MB, like your example. How do you work with the QoS ?

    You consider the maximum bandwidth 5, 3 or the sum 8Mb in the rules ?

    Any ideia ?

    Thank you very much !!

    1. Mike Post author

      In situations like this I usually have certain services tied to certain pipes anyways (and then just do the QOS based on that). For what you are asking though I would set my percentages based on the smallest pipe for safety reasons.

  5. Doug Penny

    Just to clarify, do you recommend not using a policy route for systems that need a fixed, static, external address? We have a PBX server that registers with our SIP provider and needs to have a fixed IP address. Should I not use a policy route and instead use static routes for this system?

    On a similar, but related note, how do NAT IP pools work with VIPs when using WLLB? For example, I have two VIPs setup for a server. Should I also have an IP pool setup for each VIP and use both of those when creating an outgoing traffic policy?


    1. Mike Post author


      Unless you are using BGP to route traffic from a set pool of IPs over BOTH circuits (with different weights for failover) I would definitely use policy routes to force the IP for the PBX to go out the desired pipe. If your phones phone directly out (no internal call manager) you will want to make sure they are on their own subnet for simplicity in the policy route configuration.

      In regards to the VIP Rule, the VIP will be used for incoming traffic and the return traffic will use the same external address). Now, with WLLB you can run into some weird shenanigans if the device reaches out for remote services on a regular basis. You normally want the VIP policy match parameter to be set (or a specifically policy for internal to outgoing traffic that is tied to the VIP External IP) in order to ensure that machine can reach out and be translated properly (specially mail servers etc). If pipe 1 goes down and traffic is accessed through pipe 2, the VIPs appropriate will match (and in turn the outgoing policies configured for those interfaces) would work as mentioned. Not really recommended for services that rely on the external IP to be the same for security purposes etc but it does work.

      Please let me know if I didn’t clearly explain something.

  6. Ricardo

    Mike, Sorry for commenting on an old thread. I need to make a range of ips to use a defined external interface. Lets say, I know that i can do it using policy route, but its a lot of work, when other vendors ( Sophos, by example) you can route this way in the firewall policy. I saw someone saying that i can do it using wan llb, but i was unable to do it. What do you suggest?

    1. Mike Post author

      There are WAN LLB Rules that you can enforce on 5.4 and 5.6 Beta.

      They are essentially policy routes though. The cool thing is, at least on 5.6, you are able to set the destination address as “Internet Service” and select options such as “Facebook-DNS, Facebook-FTP,Facebook-Web, Google Gmail”, and so on and so on which keeps you from having to enter individual subnets etc.

  7. Eng


    Is it possible to segregate the traffic (one link is for normal traffic for surfing another one is mainly for VPN tunnel) with 2 ISP WAN link ? my current device model is Fortigate 60D.

    Please advise the feature and mechanism to be deployed if it is feasible, Thanks

    1. Mike Post author

      You can do that fine. You just have a route that goes out your WAN1 (internet link) and then when you make your VPN Tunnel configuration you would tie it to WAN2. If you setup an route based IPSEC on WAN2 then you just setup a route saying a.b.c.d (destination network of other side of IPSEC) go out IPSEC INTERFACE and it will traverse the secondary link. If you are talking SSL VPN, you just have two routes, one uses the WAN1 link (internet) the other uses WAN2 (vpn) and you just give them WAN2 link a higher priority (wan1 0, wan2 5) so they both work using ECMP. Then just make SSL VPN listen only on the WAN2 interface.

      1. Eng

        Hi Mike,

        Thanks for your reply. Based on load balancing mechanism, I assume that it’s a mechanism in weight load balance under ECMP. Please correct me if my understanding is wrong. Thanks

        Besides, can i deploy a HA environment with two boxes 60d acts as active/standby for incoming traffic (virtual server) which to have fault-tolerance in the event of primary box is down, thanks

        1. Mike Post author

          It wouldn’t be load balancing on this one. You would just be utilizing different interfaces for different things (IPSEC on WAN2 regular internet traffic on WAN1)

  8. Lola

    Hi, I just setup the WAN LLB on my fortigate 200D. Internet connection works really well but i have challenges with SSL VPN setup and also IPSEC VPN setup.

    I have tunnels on different ISP connections. I have three ISP connections one has no VPN tunnel but the remaining two have. I have looked everywhere and cannot seem to find any documentation for setting up IPsec VPN and SSL VPN on the new fortios 5.4.. please help me… Thanks

    1. Mike Post author

      You configure the IPSEC and SSL VPN the same way you always have. In a perfect world you could set up a primary and secondary and then load balance across them with ECMP. Just set the route for the network to go out each tunnel and have it match priority and distance. Boom, your tunnel is redundant and possibly faster if the remote end has multiple connections as well.

  9. Junos

    Hi Mike,

    I have 2 WAN links having same bandwidth (80x80Mbps), for some reason I need to user PBR. I have to Bluecoat Proxies i.e. and In the PBR I created following: > WAN1 > WAN2 > WAN2
    1.1.16 > WAN1
    I need both proxies to utilize both WAN links equally always, If one proxy goes done then the other just uses the first WAN policy mentioned and doesn’t use the other to LB.

    Any suggestion how can I use both proxies to use all WAN links equally all the time?

    1. Mike Post author

      So right now all of your users use the bluecoats. The bluecoats should default to the Gate and then using WAN LLB it should auto switch. Policy routes should not be necessary to do what you are wanting. This is assuming that you have WLLB setup properly and the weights of the circuits properly set.

  10. Jose


    I have 2 wan links, a dedicated 20Mbps (fully guaranteed speed) and a 20 Mbps ADSL (20% guaranteed). I tried to use WAN LLB with the volume algorithm to properly balance the connections, but it causes several issues with internet banking sites, as it breaks SSL comm, by changing client’s IP. I changed the LB algorithm to “Source IP”, which solves my SSL problems, but now the FG is balancing the sessions evenly between the links. Is there any workaround to this? Can i use weighted algorithms and keep connections from same source ip from use different links?


    1. Mike Post author

      You can use policy routes that will give priority to a certain link (and keep it on that pipe unless it goes down)….in all honesty I much prefer to have two links tied to a zone. make my policy from inside zone to outside (wan) zone and then have health checks that pull the more quality links static route in the event of circuit failure. The crummy DSL line will just give you trouble if you try to use it live with anything else

  11. Prem

    We have 3 isp and isp1(20mbps), ips2(10mbps), isp3(4mbps). Configured spillover in fortigate os 5.4.5.
    configured like
    ISP1 –> ingress(19mbps) –> egress(19mbps)
    ISP2 –> ingress(9mbps) –> egress(9mbps)
    ISP3 –> ingress(4mbps) –> egress(4mbps)

    when i observe isp1 link gets 21MBPS used but i do not find any shift over to isp2.
    What can we configure yet to make over the shift.

    1. Mike Post author

      Is this a single machine performing a pull down of a specific file? Are you able to run multiple machines, make one go over the spillover limit and then let the second machine attempt to surf out to see if it switches to the next link?

  12. Nanthakumar Subramanian


    As Ricardo said, I have a similar situation where i want to enforce all traffics coming from a particular source subnet/IP to go via a defined WAN link to all destination. I thought policy routes would be the best option but it forces me to add more policy routes for my other requirements prior to this route.

    I am trying to achieve this by adding WAN LLB Priority rules.
    unfortunately i am unable to select users as “ALL” in the WAN LLB rules in GUI as well as in CLI.

    following are the concern;
    1. Is it mandate to have users section to use this WAN LLB priority rules?
    2. If i leave users section as blank, will this WAN LLB priority rules be effective?

    As of now i don’t have any policy routes in place and i have three WAN links which is included in WAN LLB. I use Source IP based WAN LLB.
    Firmware Version- 5.4.4

    Appreciate your response.

    Nanthakumar S


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