Cisco StackWise technology unites up to nine individual Cisco Catalyst 3750 switches into a single, logical unit. For this, Cisco StackWise uses special stack interconnect cables and stacking software. The stack behaves as a single switching unit that a master switch manages. The master switch is elected from one of the member switches. The master switch automatically creates and updates all the switching and optional routing tables. A working stack can accept new members or delete old ones without service interruption.
Consider these software compatibility recommendations for a Cisco Catalyst 3750 switch to be in a switch stack:
All stack members must run the same version of Cisco IOS Software to ensure compatibility between stack members. This ensures full compatibility in the stack protocol version among the stack members. For example, all stack members must have Enhanced Multilayer Software Image (EMI) Cisco IOS Software Release 12.1(14)EA1 installed.
If your switch stack contains switches that run Standard Multilayer Software Image (SMI) and EMI software, the switch that runs the EMI software must be the stack master. The EMI features become unavailable to all stack members if the stack master runs the SMI software.
At least two stack members must have EMI software installed to ensure redundant support for the EMI features. The EMI takes precedence over the SMI during stack master election, when the priority value of the stack members is the same. If the EMI stack master fails, the other stack member that runs the EMI software becomes the stack master.
When a switch with the EMI software joins a switch stack that runs the SMI software of the same version, the EMI switch does not automatically become the stack master. For the EMI switch to become the stack master, reset the current SMI stack master with the reload slot stack-member-number privileged EXEC command. The EMI switch now becomes the stack master, if its priority value is higher than or the same as the other stack members.