Alamo Bowl has been Big 12 vs. Pac-12 since 2010. Every game since then has involved those two conferences.
Big 12's pecking order is:
1. Sugar (vs. SEC, New Orleans - Saints stadium)
2. Alamo (Pac-12, San Antonio - Alamodome)
3. Cheez-It (ACC, Orlando - Camping World Stadium)
4. Texas (SEC, Houston - Texans stadium)
5. Liberty (SEC, Memphis)
6. Guaranteed Rate (Big Ten, Phoenix - Diamondbacks stadium)
The Sugar is required to take the winner of the Big 12 championship game, unless that team makes the CFP, in which case the Sugar is required to take the loser of the Big 12 CCG - there is no room for discretion or for the Sugar to make its own choice.
Armed Forces (American/C-USA, Fort Worth - TCU stadium) and First Responder (ACC/American/C-USA, Dallas - SMU stadium) are flex bowls that I understand are required to take a Big 12 team every now and then and would do so when the Big 12 has too many bowl eligible teams.
Iowa State has actually played in all of these bowls or their predecessors in the last 20 or so years, other than the Sugar Bowl:
*Alamo Bowl vs. Washington State
*Cheez-It Bowl vs. Notre Dame when it was Camping World Bowl
*Texas Bowl vs. TCU in 2005 when it was Houston Bowl
*Liberty Bowls vs. Memphis and Tulsa
*Guaranteed Rate Bowl vs. Pitt in 2000 when it was Insight.com Bowl