How to Add an Additional Switch to a Junos Virtual Switch Chassis
As your network expands, you may wish to add another member to the Virtual Chassis unit. You can have up to ten (10) separate physical switches in one Virtual Switch Chassis.
Start by cabling the VCPs on the rear of the third chassis. But do not power on this switch yet.
As you add switches to the Virtual Chassis, you will have to configure two of the switches to have equal mastership priority. These two switches will always serve as master and first backup. You configure the mastership priorities on the existing Virtual Chassis unit. To configure the first switch to be the master, use this command:
 user@junos-switch0# set virtual-chassis member 0 mastership-priority 255
You use the highest possible value for mastership priority to ensure that this switch continues to function as the master. Configure the second switch with the same priority so that it remains first in line to become the master:
 user@junos-switch0# set virtual-chassis member 1 mastership-priority 255
Note that this configuration will produce nondeterministic results on boot-up. It is possible that the second switch will become master if it boots first when the Virtual Chassis unit reboots. If you want switch 0 to always be master when it is available, you can set switch 0 to have a slightly higher priority than switch 1, but for both to have a higher priority than the default (128).
In this case, switch 0 will assume the master role anytime it’s available, even if switch 1 is already functioning as master and hasn’t failed. The one downside to this behavior is in a failure scenario: If switch 0 is continually rebooting, it will continue to become master, only to reboot a minute later. That’s not so good! Instead, you should use the same mastership priority for master and backup.