Juniper EX4200 Hardware Manual page 218

An active member switch might temporarily go down before coming back up as part of this procedure.
Having traffic load-balanced across member switches using a LAG helps alleviate traffic loss during this
procedure.
To add a new member switch to an existing Virtual Chassis configuration within the same wiring closet:
1. If the new member switch has been previously configured, revert that switch's configuration to the
factory defaults. See Reverting to the Default Factory Configuration for the EX Series Switch.
2. Interconnect the unpowered new switch to one member of the existing Virtual Chassis configuration
using the dedicated Virtual Chassis ports (VCPs). Connect only one VCP on the unpowered new switch
to a VCP on a member switch in the existing Virtual Chassis at this point of the procedure.
3. Power on the new switch.
4. Confirm that the new member switch is now included within the Virtual Chassis configuration by
checking the front-panel LCD for the member ID. It should display a member ID that is greater than 0
(1 through 9), because there is already at least one member of the Virtual Chassis configuration.
NOTE: If you are using a preprovisioned configuration, the member ID is automatically
assigned to the member's serial number in the configuration file.
5. Cable the other dedicated VCP on the new member switch to the Virtual Chassis.
CAUTION:
Virtual Chassis at the same time, a member switch that was already part of the
Virtual Chassis might become nonoperational for several seconds. Network traffic
to this switch is dropped during the downtime.
The member switch will return to the normal operational state with no user
intervention, and normal operation of the Virtual Chassis will resume after this
downtime.
If you immediately cable both VCPs on the new switch into the existing
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