If i have a cast a quickened or sustained spell on myself and sombody wants to break it, can i defend actively agains it, which means to "protect" the spell besides what it can do for itself?

No. But even without "actively" defending against dispelling, you still get your Magic and the spell's Force (and the karma that you spent on it for quickened spells) vs. the enemy mage's counterspelling and Magic. And you are instantly aware of the attempt.

Just remember to watch out for the troll with the sniper rifle.

???

It's from the opening fiction to SR2.

"Once the magician lost his force field, he was vulnerable to Smedley's sniper rifle. End of story."

Admitedly, the circumstances were slightly different but the truisim holds, I think.

I'm waiting for magic to catch up with technology. There are:

Electronic measures :: Spells

Electronic counter measures :: Counterspelling

Electronic counter-counter measures :: ??

In response, Force of Will