[CentOS] Network switch to IPV6 after PXE boot

I'm trying to autoinstall a DELL 9030 desktop with CentOS7 using PXE and
Kikstart. I fall in a strange problem (I have several other laptop or
desktop using this install process without any problem, kernel and
initrd are from a Centos 7.4 iso)

- PXE boot start and the desktop load the kernel and the inirtd file

- the kernel is booted and at this time the led of the network switch
goes down for the port used by the desktop.

- then I have a message "no carrier detected on interface em1"

- followed by " IPv6 ADDRCONF(NETDEV_CHANGED): em1: link become ready
(and of course the led goes back to green on the switch)

But I have no IPV6 network setup!

How can I force to IPV4 setup to load the kikstart file ?

I've tried do switch from legacy boot to UEFI boot (my local server
setup can answer requests for the two options), update the BIOS of the
desktop...


Any help is welcome


Patrick