The DC requests to mount /tftpboot/<IP address of DC>
(in 2.1 and above: /tftpboot/<name of DC in bootptab>)
as its / by NFS from server. You must export this from the
server (rw, no_root_squash) because the DC wants to write on it
(log files, etc).
The / must contain /sbin, /bin, /lib, /etc, /var, /tmp,
/root, /dev and /proc.
/sbin, /bin, /lib/ can be a copy of an existing RH5 system. They
can be shared between all DCs. But hard links only. BTW, don't link to
server originals.
/etc, /var and /dev should be
non-sharable copies. Customise /etc/sysconfig/network,
/etc/sysconfig/network-scripts/ifcfg-eth0, /etc/fstab,
/etc/conf.modules, and others. Turn off all network services you
don't need. Remove all stuff you don't need from /var, e.g.
RPM db, lpd files.
/root and /proc should just exist. /tmp
should exist and be mode 1777.
You probably want to create /usr and /home mount
points. /usr can be mounted ro.
About 10 MB per DC plus about 15 MB of shared files should be sufficient. BTW: if your DCs are quite similar, the kernel image can also be shared.
Here is an illustrative script to create the first root filesystem. Here is an illustrative script to duplicate the root filesystem.