This article explains how does the default load balance work, and what's the difference between "Auto Weight" and "According to Line Speed" mode.
For multi-WAN models, by default, the Vigor Router will balance the traffic load across all the online WAN interfaces. The router makes the routing decision according to the ratio of each WAN's remaining bandwidth, which is calculated by subtracting the bandwidth currently used from the maximum bandwidth.
For example, suppose there are two WAN interfaces online, WAN1 and WAN2. WAN1's maximum bandwidth is 10Mpbs, and WAN2 is 20Mbps. If both WAN have 5Mbps traffic at the moment, then the remaining bandwidth of WAN1 is 5Mbps, and WAN2 is 15Mbps. The ratio of the remaining bandwidth of WAN1 to WAN2 is 1:3, so the load balance weight of WAN1 and WAN2 will be 1:3. Which means if now there are four new sessions, the router will send one of them to WAN1, and three of them to WAN2.
And the router will keep monitoring the bandwidth used on each WAN interface, and update the load balance weight periodically.
On WAN >> General Setup page, there are two options for Load Balance Mode: Auto Weight and According to Line Speed. The difference between them is how does the router determine the "maximum bandwidth" of each WAN interface.
The default setting is "Auto Weight." In this mode, the maximum bandwidth automatically calculated by the router according to the bandwidth peak that the WAN interface has reached recently. In "According to Line Speed" mode, the maximum bandwidth is the Line Speed value defined by the network administrator on WAN >> General Setup >> WAN index page. (Therefore, if you'd like to use According to Line Speed mode, don't forget to set the Line Speed for each WAN.)
In summary, if the WAN interface has a dynamic line speed, use Auto Weight to allow the router to adjust the load balance weight according to the line status. If the bandwidth is fixed, use According to Line Speed mode will make sure the router won't underestimate the available bandwidth.
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