Dell PowerEdge FN I/O Module Configuration Guide 9.10(0.0)

The recommended procedure for merging two operational stacks is as follows:

1. Always power off all units in one stack before connecting to another stack.
2. Add the units as a group by unplugging one stacking cable in the operational stack and physically connecting all unpowered units.
3. Completely cable the stacking connections, making sure the redundant link is also in place.

Two operational stacks can also be merged by reconnecting stack cables without powering down units in either stack. Connecting a powered-up standalone unit to an existing stack leads to same behavior as when merging two operational stacks. In such cases, Manager re-election is done and the Manager with the higher MAC address wins the election. The losing stack manager resets itself and all its member units. After the reset, all the losing stack members join the winning stack to form a single stack. The winning stack remains functional through the merge process. If the stack merge is performed in this way, then it is strongly recommended that the user set the admin management preference of the desired winner stack manager to a higher value than the stack manager that should lose the election.