Hola

De acuerdo con el documento de Q&A de los modelos 3750X, el hardware mix esta soportado:

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps6406/qa_c67-578933.html

Q. Are mixed stacks supported?

A. It depends. The Cisco StackWise and StackWise Plus solutions are flexible enough to allow you to:

• Stack any Cisco Catalyst 3750 switch in the same stack (hardware mixed stack)

• Stack any Cisco IOS Software feature set in the same stack (software mixed stack)

One  exception is the new LAN Base models being introduced with the Cisco  Catalyst 3750-X Series, which can stack only with other Catalyst 3750-X  LAN Base models.

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Por favor asegurate que la licencia del 3750-X no sea LAN BASE, ya de acuerdo con la documentacion no se va a comunicar con el stack

Saludos

Adolfo Cabrera