my ISP informed me that the gigabit router has arrived and tomorrow I'll be installing it
thus, my little experiment will be stopped
the 605L will be used as an wireless AP, no routing at all; and will also turn off the SPI Firewall and the DHCP server - if with this config the unit still proves problematic ...
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if I find some person willing, I would test the router in a different environment
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note:
for the last week I've been manually clearing the log multiple times a day and found out that the router stability is not degrading - looks pretty conclusive to me