Fortigate Load balancing doesn't work.

[arnaudnpago]

Hello to all,

I've configured a load balancing between 2 interfaces, by setting one to 62% and the other to 38% of the internet trafic. But it does not seem to work well, indeed I notice that the traffic is not shared as configured above.
I use a Fortigate 200D, Firmware version: v5.4.1, build1064 (GA).
I would like to know how to fix this problem please.

Your help will be very appreciated.

[gfleming]

What are the capacities of the two links today?

And please share the metrics you are using the measure the effecivejness of the load balancing. How are you confirming this is not working?

[arnaudnpago]

Hello,
Sorry for the delay of my answer. 

- I've two interfaces named: Wan2 (that carries an internet bandwith of 16M) and port15 (that carries 10M)

- Based on the information above, I set the load balancing as the image below shows: 

- Below is the result of the bandwidth consumption per interface:

The image shows us that the percentages are not effective in the result.

[gfleming]

OK so you have to consider something here. The load balancer is balancing sessions, not packets. So it will do its best to maintain a given ratio as you specify. However if you have a small number or sessions with a high bandwidth utilization it will skew the ratio a bit.

The balancing is working if you look at your graph. i.e. both links are saturated between 12:00-12:15.

The firewall is doing its best to push sessions on to the appropriate link based on utilization ratios.

[arnaudnpago]

Ok well received.
I thought my load balancing Algorithm was volume based (as the image above suggests). 

Thanks

[gfleming · Accepted solution]

It is volume based. But as I mentioned it's not possible to absolutely get it 100% accurate because of the way traffic flows work. The FGT will do its best but it won't always be perfect. If you have hundreds of sessions it will get closer to the approximation of what you've specified in the config. If you only have a handful of sessions it won't be as easy to match the ratio.

[fred]

FortiOS 5.4.1 is very very old, you may want to upgrade if possible, and even set up SD-WAN with loadbalancing. It comes in many flavors..