#yum install xl2tpd
complete

#systemctl enable xl2tpd
No issue

#systemctl start xl2tpd
A message:
Job for xl2tpd.service failed. See 'systemctl status xl2tpd.service' and 'journalctl -xn' for details.

#systemctl status xl2tpd.service
A message:
xl2tpd.service - Level 2 Tunnel Protocol Daemon (L2TP)
Loaded: loaded (/usr/lib/systemd/system/xl2tpd.service; enabled)
Active: failed (Result: start-limit) since Fri 2015-06-26 08:25:27 UTC; 7s ago
Process: 7011 ExecStartPre=/sbin/modprobe -q l2tp_ppp (code=exited, status=1/FAILURE)

Why there's a start-limit? What can I do to start the l2tpd service?

Thank you in advance!

I'm not a systemd guy, but it looks like the start script is failing because the modprobe command fails. This is probably because you are using the Linode kernel, which has most config options built in rather than loadable as modules. If this is the case, you will either have to switch to using another kernel (such as the one provided by your distro), or change the start script so it does not try to run modprobe.

Thanks Vance!
I will let you know the result later after issue solved.

Vance,the issue solved by this way:provided by your distro)

the article has one problem at #uname -a

The point is the replacements must be exactly same as the related file name in /boot/