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Thank you for asking this question in the support community. There is a good explanation of your question in the Configuring ActivChassis in AOS guide starting on page 37.
A Device with VCID 1 Boots in ActivChassis Mode
If a device with a VCID of 1 boots in ActivChassis mode, the manifest is consulted and all ports configured for ActivChassis are enabled. An ActivChassis detection period is invoked to wait for all units in the manifest to be detected. During this process, if Escape is selected, the device stops detecting, and elects a master from the currently detected master candidates. If elected master, it then restores from its local startup configuration file. If Escape is not selected, the device continues detection and an election takes place to verify the device’s role or determine the ActivChassis master. If another device with VCID 1 is detected, the unit will display an error stating that duplicate master candidates exist, and boot immediately with no linecard attached. the unit will stay in this state until the duplicate is removed and a valid election can take place.
If the device is elected master, it restores from its local startup configuration. As new devices request to join the ActivChassis, the master device updates its manifest with information about the new member, assigned VCID configuration is applied to the new device, and the file systems are updated and synchronized. As previously joined devices return, they are admitted to ActivChassis if they still meet admission requirements, and their hardware configuration matches what was previously recorded in the manifest.
If a device with VCID 2 is already the active master, the master candidate (VCID 1) is added as a non-master device to the working ActivChassis. If the device is elected as the backup device and is admitted to the ActivChassis, it is configured by the master device and becomes the backup. If this device was already fully booted and master of itself or some set of linecards, it becomes a case of ActivChassis merge.
A Device with VCID 2 Boots in ActivChassis Mode
If a device with the VCID of 2 boots in ActivChassis mode, it operates in the same manner as the previous scenario, except that if VCID 1 and 2 are detected at master election, the elected master will be whichever unit was the previous master of the ActivChassis. If both claim they were the previous master, then VCID 1 wins, and VCID 2 becomes the active backup.
An analogy is the ActivChassis VID is a priority (lowest most preferred) that does not preempt.
The rest of your network design should use common design practices, where ports connected to other switches are utilizing spanning-tree protocol (STP), and configured as trunk ports. Please, do not hesitate to reply to this post with any additional questions you have. I will be happy to help in any way I can.
Levi
