How should the crashkernel parameter be configured for using kdump on RHEL7?

Issue

• When configuring the crashkernel parameter the kdump service fails to start:

$ systemctl start kdump
Job for kdump.service failed. See 'systemctl status kdump.service' and 'journalctl -xn' for details.
$ systemctl status kdump.service
kdump.service - Crash recovery kernel arming
   Loaded: loaded (/usr/lib/systemd/system/kdump.service; enabled)
   Active: failed (Result: exit-code) since Wed 2014-06-11 10:34:48 CEST; 3min 10s ago
  Process: 2259 ExecStart=/usr/bin/kdumpctl start (code=exited, status=1/FAILURE)
 Main PID: 2259 (code=exited, status=1/FAILURE)

Jun 11 10:34:47 rhel7 systemd[1]: Starting Crash recovery kernel arming...
Jun 11 10:34:48 rhel7 kdumpctl[2259]: No memory reserved for crash kernel.
Jun 11 10:34:48 rhel7 kdumpctl[2259]: Starting kdump: [FAILED]
Jun 11 10:34:48 rhel7 systemd[1]: kdump.service: main process exited, code=exited, status=1/FAILURE
Jun 11 10:34:48 rhel7 systemd[1]: Failed to start Crash recovery kernel arming.
Jun 11 10:34:48 rhel7 systemd[1]: Unit kdump.service entered failed state.

• Unable to start/restart kdump service
• What is the correct crashkernel parameter for kdump to work?
• "crashkernel reservation failed - memory is in use" errors when kernel panics