[FS#253] Multicast over wireless ceases to work properly after a while on ath9k with many clients connected

[LEDE Bugs]

The following task has a new comment added:

FS#253 - Multicast over wireless ceases to work properly after a while on ath9k with many clients connected
User who did this - Neil Crawforth (neil)

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I suspect this bug is responsible for my Chromecasts disappearing every day or so, since they use multicast for discovery.

I have a GL-iNet 6416A (Atheros AR9330 rev 1) with about 13 wireless clients. I maintain 2 other identical routers for friends, with 2 or 3 wireless clients each, which don't seem to suffer from the same problem.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=253#comment796