I don't think it belongs in ifconfig. I would suggest looking in the NetBSD gudie, and perhaps sending a patch/snippet to improve that. I'd say that ifocnfig.XX and rc.conf are both traditional ways and it's personal preference. My only hard suggestion would be pick one, don't mix. Basically the rc.d script reads our of file or variables and calls ifconfig - there isn't much more to it.
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