avm fritzbox uses very long RA period so it effectively only
responds after a router solicitation. when there are multiple
fritzbox routers on the lan, then while configuring one prefix
of the first RA, the ip stack can drop the second router
advertisement and we would never get the second route.
packets can always get lost. so we just keep on sending router
solicitations (up to 3 times) to make sure we got all the RA's.
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