Ziegler, 31, is under team control through 2014 and is owed only $405K for the balance of the season. He has a 2.39 ERA, 6.9 K/9, 3.1 BB/9, and 71.6% groundball rate with no home runs allowed in 37 2/3 innings this year. As you might expect, he's a lot tougher on right-handed hitters.
Allen, 25, hasn't done much in 209 scattered big league plate appearances since coming to Arizona two years ago in a trade with the White Sox. He's logged a lot of time in Triple-A, and the Pacific Coast League is hitter-friendly, but he is hitting .306/.427/.579 there this year.
Norberto, 24, tossed 20 ugly big league innings last year and has been in Triple-A this year. The lefty has a 4.25 ERA, 10.0 K/9, 4.8 BB/9, and 0.2 HR/9 in 48 2/3 innings. He has good velocity for a southpaw and ranked 29th among D'Backs prospects before the 2010 season, according to Baseball America.