Comment # 2 on bug 1173402 from Thomas Blume

(In reply to Fabian Vogt from comment #0)
> @dracut maintainers: It seems like "rd.neednet=1" is enough to get dracut to
> acquire an address over DHCP, so a workaround is to just drop "ip=dhcp". I'd
> like to have confirmation that this is intended behaviour and won't break
> without notice in the future. This doesn't help for cases without DHCP
> though.
>
> If it's not possible to avoid the "ip=foo" induced kernel delay, it might be
> a good idea to provide "rd.ip=foo" or similar as alias, which is ignored by
> the kernel.

Need to double check but AFAIK, rd.neednet isn't sufficient, at least if the
machine has multiple network interfaces. In this case you need to tell dracut
which interface to use.
ip=dhcp works in this case, because it is a wildcard to try all network
interfaces.