It occurred to me that Jonathan A. Kollasch wrote in gmane.os.netbsd.current:
> For me (running under /amd64), it always comes back, after 5-15 minutes.
> And it does seem it may be caused by some traffic patterns.

For me (Asus A8N-E, /amd64), it never comes back by itself - but dhclient
somehow gets the job done, and the IP alias I use to access my modem will
continue to work.