i am working on fortigate 401e configuration and i have 4 different internet ISP connection, could fortigate 401e merge the 4 connections bandwidth to increase the upload and download bandwidth and be the sum of the 4 lines bandwidth?
It wouldn't be possible to achieve combined speed of 4 ISP on a single session/stream. Even if you have 4 ISP links ,they are logically separate. Each and every ISP link will be having a bandwidth cap limit provided by the ISP which will be able to handle only that amount of traffic/BW
Increasing the bandwidth for a single session is not possible.
What you could achieve is to distribute sessions equally to all ISPs, thus maximizing the combined bandwidth.
In order to do so, create an SD-WAN zone, place all 4 WAN interfaces into it, and create an SD-WAN rule. Usually, one would prefer the ISP which has the biggest bandwidth, highest stability or lowest latency. In your case, you can configure that SD-WAN members will be used in a round-robin fashion, that is, each new session is handed to the next available WAN interface. You can even put weights onto the connections reflecting the bandwidth they provide.
For details, please read up the SD-WAN chapter in the Admin Handbook.
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