How can I configure the network interface on Linux to automatically reconnect if the cable was disconnected.?

Are you talking about reconnecting to the network or re-authenticating with AD?

If you are using a static IP it will reconnect instantly  when the cable is plugged back in.

I am not sure about how AD handles lost connections.

I am talking about reconnecting to the network.  I am set up to use DHCP and if I disconnect the cable then reconnect it, I have to manually activate the connection again.  I am wondering if there is a way to have the OS auto reconnect to the network.  The AD part is only related in the sense that I can't log back in and manually activate the network interface because my AD credentials can't be authenticated since AD is unreachable.   I hope this explanation is clear.

you might want to taking a look @ dhclient.conf

here is some information on users having issues switching between connections -

http://ubuntuforums.org/archive/index.php/t-43766.html

You may want to try running a script to check network connectivity every minute:

#!/bin/bash
if ! `ping -c5 192.168.1.1 >/dev/null 2>&1` ; then
/etc/rc.d/init.d/network restart
fi
exit 0

This would perform 5 ping test and if it fails, it will restart the network.

You can save it in /usr/bin/pingtest then chmod 775 /usr/bin/pingtest to make it executable

Using crontab -e, input:

* * * * *  /usr/bin/pingtest

Verify that the script is working, once verified, stay logged in first then unplug your network cable then plug it again...