[CentOS] what is difference between ifconfig and ifup?

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"?

thanks.