Find the ifup-post under /etc/sysconfig/network-scripts. This scipt is called right after any network interface is brought up online. In this script, you will find the following code snippet toward the end.
if [ -x /sbin/ifup-local ]; then
    /sbin/ifup-local ${DEVICE}


In the code snippet above, if ifup-local script exists in /sbin location, then script gets executed with an interface name in argument. Usually no such ecript like ifup-local exists so in order to run a startup script automatically after a network interface is up. Create an executable script called ifup-local in /sbin and put in there any command or script you wish to run.

Here is an example:

if [[ "$1" == "eth0" ]]
then
  echo "this part will be executed right after eth0 is up."
  echo "so you can put any startup command for eth0 here"
else
  #DO_NOTHING
fi


when script is done, use command to get the script executable.


$ sudo chmod +x /sbin/ifup-local
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