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3750 Stackwise using mixed versions

I was unsure if it was possible to create a 3750 stack using a mixture of standard and enhanced licenses.  I was pretty sure they would join the stack but was unsure what features would work afterwards.  Would the entire stack gain routing functionality or would it be limited to just the EMI images?  Would the entire stack be forced to run as an SMI image?  These were the questions I needed answers to.  After much digging I came across a Cisco TAC article which answered all my questions.
* The IOS software version on all stack members, including the stack master, should be the same. This helps ensure full compatibility in the stack protocol version among the stack members. For example, all stack members should have either the EMI or SMI

* If your switch stack must have switches running SMI and EMI software,the switch running the EMI software should be the stack master. EMI features become unavailable to all stack members if the stack master is running the SMI software.

* At least two stack members should have the EMI software installed to ensure redundant support of the EMI features. The EMI has precedence over the SMI during stack master election, assuming that the priority value of the stack members are the same. If the EMI stack master fails,the other stack member running the EMI software becomes the stack master.

* When a switch running the EMI joins a switch stack running the SMI of the same version, the EMI switch does not automatically become the stack master. If you want the EMI switch to become the stack master, reset the current SMI stack master by using the reload slot stack-member-number privileged EXEC command. The EMI switch is elected the stack master, assuming its priority value is higher or the same as the other stack members.

However, there are some issues to consider when deploying such mixed stacks.

* If you have a stack of SMI switches, with a single EMI unit, the EMI unit will, by default, become the master switch. It will give EMI functionality to the entire stack. This is because Layer-3 functionality is centralized in nature, on the 3750. However, if the master switch were to fail, the entire stack would revert to the feature-set of the new master, who happens to be SMI. The user would lose all EMI-only functionality such as advanced IP routing protocols. This is the reason we include a recommendation that if users deploy a mixed stack, they should have at least two EMI units to provide protection against such failures.

* If the user does decide to use the unified upgrade feature to upgrade the stack, it will be a single upgrade. This means that the entire stack will get upgraded to the same image. The upgrade process will involve copying the same new image file to the flash of every switch in the stack. So, if the new image is an EMI image, all the EMI switches will get upgraded. The originally SMI switches will not get upgraded to EMI, in this scenario! They will give a "Version Mismatch" error and you will need to upgrade them separately, to an SMI image (or purchase an SMI-to-EMI upgrade license for those switches).

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