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