[CentOS] what is difference between ifconfig and ifup?

Mon Feb 3 21:30:35 UTC 2014
mcclnx mcc <mcclnx at yahoo.com.tw>

I and testing command line to up and down ethernet connection.

if I perform following, client can not re-connect.
      ifconfig eth0 down
      ifconfig eth0 up

if I use following, client can re-connect:
      ifconfig eth0 down
      ifup eth0

What difference between "ifconfig up" and "ifup"?