How to specify the DHCP IP address ? (CentOS 5.3)

I found out the reason why it asks for a lease before the link is ready. It is because ifup-dhcp does try to wait until the interface is ready before calling dhcpcd, but it uses the function is_iface_up which is defined in /etc/sysconfig/network/scripts/functions, and that function only checks to see if eth0 exists before returning a positive result.

I also found a (slightly) more long term solution. ifup-dhcp waits for a positive result from is_iface_up and then it waits an additional amount of seconds as defined by DHCLIENT_SLEEP which can be found in /etc/sysconfig/network/dhcp. By setting DHCLIENT_SLEEP="3", the interface now has time to become ready before dhcpcd is called, despite the fact that is_iface_up returns too soon. This is also less likely to be clobbered by updates.

I don't know the answer to your question, but I just want to warn you about using the DHCP server address as your default router. If that works it is by chance... In IPv6 the default route is communicated by RA's and not by DHCP.

You will need to enable forwarding, accepting RA's and sending out RS's. A problem here is that the kernel included in Centos6 does not allow you to do that. When forwarding is enabled then accepting RA's and sending RS's is disabled. There should be a CentosPlus kernel with the relevant patches though. See the mailing list thread / message at http://lists.centos.org/pipermail/centos/2011-April/thread.html#109756

The relevant sysctl.conf settings are:

net.ipv6.conf.all.forwarding=1
net.ipv6.conf.eth0.accept_ra=2
net.ipv6.conf.eth0.accept_ra_defrtr=1
net.ipv6.conf.eth0.router_solicitations=1