Understanding How the Master in an EX4200 or EX4500 Virtual Chassis Is Elected

All switches that are interconnected in an EX4200 or EX4500 Virtual Chassis configuration are member switches of that Virtual Chassis. Each Virtual Chassis configuration has one member that functions as the master and controls the Virtual Chassis configuration.

When a Virtual Chassis configuration boots, the Juniper Networks Junos operating system (Junos OS) for the switches automatically runs a master election algorithm to determine which member switch takes the role of master.

The algorithm that the software uses to determine the master is as follows:

1. Choose the member with the highest user-configured mastership priority (255 is the highest possible value). A switch with a configured mastership priority of 0 will always stay in the linecard role.
2. Choose the member that was master the last time the Virtual Chassis configuration booted.
3. Choose the member that has been included in the Virtual Chassis configuration for the longest period of time. (For this to be a deciding factor, there has to be a minimum time lapse of 1 minute between the power-ons of the individual interconnected member switches.)
4. Choose the member with the lowest MAC address.

The variations among switches and switch models do not impact the master election algorithm.

To ensure that a specific member is elected as the master:

1. Power on only the switch that you want to configure as master of the Virtual Chassis configuration.
2. Configure the mastership priority of that member to have the highest possible value (255).

   Note: In a mixed EX4200 and EX4500 Virtual Chassis that is not preprovisioned, you must assign the highest possible value to two EX4200 or EX4500 switches to ensure the same type of switch is acting in the master and backup roles. The same type of switch must be in the master and backup roles in a mixed EX4200 and EX4500 Virtual Chassis. We also recommend a mastership priority of 0 for linecard member switches in a mixed EX4200 and EX4500 Virtual Chassis in most configurations. A mastership priority of 0 ensures that switches in the linecard role never assume the master role.

3. Continue to configure other members through the master member.
4. Power on the other members.