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 Stack Creation and Modification
Stacks are created when individual switches are joined together with stacking cables. When the stack ports detect electromechanical activity, each port starts to transmit information about its switch. When the complete set of switches is known, the stack elects one of the members to be the master switch, which will be responsible for maintaining and updating configuration files, routing information, and other stack information. The entire stack will have a single IP address that will be used by all the switches.

Online Stack Adds and Removals
Switches can be added and deleted to a working stack without affecting stack performance. When a new switch is added, the master switch automatically configures the unit with the currently running Cisco IOS® Software image and configuration of the stack. The stack will gather information such as switching table information and update the MAC tables as new addresses are learned. The network manager does not have to do anything to bring up the switch before it is ready to operate. Similarly, switches can be removed from a working stack without any operational effect on the remaining switches. When the stack discovers that a series of ports is no longer present, it will update this information without affecting forwarding or routing.

Master Switch Election
The stack behaves as a single switching unit that is managed by a master switch elected from one of the member switches. The master switch automatically creates and updates all the switching and optional routing tables. Any member of the stack can become the master switch. Upon installation, or reboot of the entire stack, an election process occurs among the switches in the stack. There is a hierarchy of selection criteria for the election.

1. User priority-The network manager can select a switch to be master.
2. Hardware and software priority-This will default to the unit with the most extensive feature set. The Cisco Catalyst 3750 Advanced IP Services IPv6 (AIPv6) image has the highest priority, followed by Cisco Catalyst 3750 switches with Enhanced Multilayer Software Image (EMI) and then the Standard Multilayer Software Image (SMI) versions.
3. Default configuration-If a switch has preexisting configuration information, it will take precedence over switches that have not been configured.
4. Uptime-The switch that has been running the longest is selected.
5. MAC address-Each switch reports its MAC address to all its neighbors for comparison. The switch with the lowest MAC address is selected.

Adding New Members
When the switching stack has established a master, any new switch added afterward automatically becomes a subordinate. All the current routing and addressing information is downloaded into the subordinate so that it can immediately begin transmitting traffic. Its ports become identified with the IP address of the master switch. Global information, such as QoS configuration settings, is downloaded into the new subordinate member.

Cisco IOS Software Images Must Be Identical
The Cisco StackWise technology requires that all units in the stack run the same release of Cisco IOS Software. When the stack is first built, it is recommended that all of the stack members have the same level of software - either all SMI, all EMI, or AIPv6. This is because later upgrades of Cisco IOS Software mandate that all the switches to be upgraded to the same version as the master.
Automatic Cisco IOS Software Upgrade/Downgrade from the Master Switch
When a new switch is added to an existing stack, the master switch communicates with the switch to determine if the Cisco IOS Software image is the same as the one on the stack. If it is the same, the master switch sends the stack configuration to the device and the ports are brought online. If the Cisco IOS Software image is not the same, one of three things will occur:

1. If the hardware of the new switch is supported by the Cisco IOS Software image running on the stack, the master will by default download the Cisco IOS Software image in the master's Flash memory to the new switch, send down the stack configuration, and bring the switch online.
2. If the hardware of the new switch is supported by the Cisco IOS Software image running on the stack and the user has configured a Trivial File Transfer Protocol (TFTP) server for Cisco IOS Software image downloads, then the master will automatically download the Cisco IOS Software image from the TFTP server to the new switch, configure it, then bring it online.
3. If the hardware of the new switch is not supported by the Cisco IOS Software image running on the stack, the master will put the new switch into a suspended state, notify the user of a version incompatibility, and wait until the user upgrades the master to a Cisco IOS Software image that supports both types of hardware. The master will then upgrade the rest of the stack to this version, including the new switch, and bring the stack online.