[FS#600] Random disconnection on 17.01

[LEDE Bugs]

The following task has a new comment added:

FS#600 - Random disconnection on 17.01
User who did this - Emmanuel Lepage Vallée (Elv13)

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After a few days, I can see that WAN also disconnect randomly for a while. The ram usage never reach critical level (~22% free when it happen)
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=600#comment2084