Ubuntu 20.04 LTS does not boot up using PXE and NFS

Question

To allow me to test Ubuntu Live I have been using same setup for the last 10 Years

PXE server using Debian

/srv/tftp/pxelinux.cfg/default
  LABEL Ubuntu 18.04 Live
  kernel ubuntu18/casper/vmlinuz
  append boot=casper netboot=nfs nfsroot=10.11.97.5:/srv/tftp/ubuntu18 initrd=ubuntu18/casper/initrd

Download iso live desktop and mount locally:

/etc/fstab:
/srv/tftp/ubuntu-18.04.4-desktop-amd64.iso /srv/tftp/ubuntu18     iso9660 loop,ro 0 0

ISO exported using nfs:

/etc/exports
/srv/tftp/ubuntu18      *(async,no_root_squash,no_subtree_check,ro)

but somehow ubuntu-20.04-desktop-amd64.iso (2020-04-23) does not connect to the nfs share. connect: Network is unreacheable

NFS over TCP not available from 10.11.97.5
done.
Unable to find a live file system on the network 

Looks like the interface does not bring up. If I compare the 18.04 and 20.04 releases, on the 20.04 it does not ask for an ip. IP-Config: lines missing.

Solution

I just found that a new parameter ip=dhcp is needed since 19.10 https://bugs.launchpad.net/ubuntu/+source/casper/+bug/1848018