ifup and ifdown now handle pump and dhcpcd, using whichever client is installed, giving preference to pump. It would be nice if the scripts also permitted the use of the new ISC reference client dhclient. An RPM for this client is available from ISC's DHCP webpage.
As we don't currently ship dhclient, we wouldn't add support to the scripts for it.
The latest DHCP RPM, dhcp-3.0rc12-2cra.src.rpm, will patch ifup and ifdown for dhclient support. Note that Red Hat DOES ship an older ISC DHCP server, version 2. The newest server (version 3) ships with dhclient, sharing the same code base. The client and server are packaged as two subpackages of the common dhcp source RPM.