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Understanding How the Primary in a Virtual Chassis Is Elected


This topic does not apply to EX8200 Virtual Chassis.

All switches that are interconnected in a Virtual Chassis configuration are member switches of that Virtual Chassis. Each Virtual Chassis configuration has one member that functions as the primary in a Routing Engine role and controls the Virtual Chassis configuration.

When a Virtual Chassis configuration boots, it automatically runs a primary-role election algorithm to determine which member switch assumes the role of primary. The algorithm compares the Virtual Chassis members against the following criteria (in the order listed) until only one member remains under consideration, and that member becomes the primary:

  1. Choose the member with the highest primary-role priority.

    Members have primary-role priority 128 by default, and 255 is the highest possible value you can manually configure. A member with primary-role priority 0 will never be elected as primary (or backup), and always stays in the linecard role.

  2. Choose the member that was primary the last time you rebooted the Virtual Chassis.

  3. Choose the member that has been in the Virtual Chassis configuration for the longest period of time.

    (The member switches under consideration must have more than 1 minute between power-up times for this condition to make a difference.)

  4. Choose the member with the lowest MAC address.

The primary-role election algorithm does not consider the different switch models or platforms in the Virtual Chassis. For some Virtual Chassis that can contain different types of switches, we require or recommend you configure certain switches in the primary and backup Routing Engine roles. See Understanding Mixed EX Series and QFX Series Virtual Chassis for details on the types of switches that can be mixed in a Virtual Chassis and which switches can or must be the primary or backup members.

To make sure a specific member is elected as the primary:

  1. Power on only the switch that you want to configure as primary of the Virtual Chassis configuration.

  2. Configure the primary-role priority of that member to have the highest possible value (255).

  3. Continue to configure other members through the primary member.

  4. Power on the other members.

You usually want to assign the same (highest) primary-role priority value to the members you want to be the primary and backup Routing Engine members to ensure reliable graceful Routing Engine switchover (GRES) operation. The other election considerations also help keep primary role from switching back and forth rapidly between the two members under failover conditions.

You can also specify the switch roles by preprovisioning your Virtual Chassis. When you preprovision a Virtual Chassis, you manually assign the member ID and role for each switch in the Virtual Chassis. See Configuring an EX3300 Virtual Chassis (CLI Procedure), Configuring an EX4200, EX4500, or EX4550 Virtual Chassis (CLI Procedure), Configuring an EX2300, EX3400, or EX4300 Virtual Chassis, or Configuring an EX4650 or a QFX Series Virtual Chassis.