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Virtual Chassis Overview for Switches

Many Juniper Networks EX Series and QFX Series switches support Virtual Chassis, a flexible and scalable technology with which you can connect individual switches together to form one unit, and configure and manage the unit as a single chassis. Virtual Chassis ports (VCPs) connect member switches together to form a Virtual Chassis, and are responsible for passing all data and control traffic between member switches.

Use the following links to find the right Virtual Chassis User Guide for different EX Series and QFX Series switches if this guide doesn’t cover the switches you’re interested in:


Virtual Chassis Fabric (VCF) is an evolution of Virtual Chassis technology. VCF configurations share some elements of Virtual Chassis technology, including configuration statements and administrative commands, but use a spine-and-leaf topology with supported QFX Series switches as spine devices and supported EX Series and QFX Series switches as leaf devices. For details on configuring and maintaining a VCF, see the Virtual Chassis Fabric User Guide.

Benefits of Virtual Chassis on Switches

  • Simplifies configuration and maintenance: Multiple devices can be managed as a single device with the same or similar capabilities as the standalone device.

  • Increases fault tolerance and high availability (HA): A Virtual Chassis can remain active and network traffic can be redirected to other member switches when a single member switch fails.

  • Flattens your network and reduces networking overhead by allowing network devices to synchronize to one resilient logical device rather than to multiple physical devices.

  • Enables a simplified Layer 2 network topology that minimizes or eliminates the need for loop prevention protocols such as Spanning Tree Protocol (STP).

  • Provides a flexible model for expanding your network: You can easily add Virtual Chassis member switches to increase the number of access ports on your network to support more servers, computers, phones, or other devices with minimal complications to the existing network topology and switch configuration.

Virtual Chassis Basics on Switches

Virtual Chassis is a feature on Juniper Networks routing or switching devices that provides network resiliency in the form of redundant Routing Engines and network expansion flexibility with minimal impact to a configuration.

Virtual Chassis technology on switches enables you to interconnect supported combinations of EX Series and QFX Series switches into one logical device that you can configure and manage as a single unit. Switches interconnected into a Virtual Chassis are called Virtual Chassis member switches, identified by a member ID within the Virtual Chassis.

Virtual Chassis member switches are interconnected and communicate with each other using Virtual Chassis ports (VCPs).

Connecting Member Switches with Virtual Chassis Ports

A switch is not recognized by the Virtual Chassis as a member switch until it is interconnected with the primary or interconnected with an existing member of the Virtual Chassis using VCPs. EX Series and QFX Series switches that can be in a Virtual Chassis might support one or more of the following VCP options:

  • Network or uplink ports that support the option to be configured as VCPs. Most switches support this VCP option.

  • Ports that are configured as VCPs in the default factory settings. These ports can also be converted into and used as network ports instead of as VCPs, and converted back into VCPs again if needed.

  • Dedicated VCPs, which are ports that can only function as VCPs. Only a few switches have dedicated VCPs.

Available VCP options vary among the different switch models. See Virtual Chassis Port Options for a summary of the ports that are supported as VCPs on different switches.

When a port is set as a VCP, it cannot be used for any other purpose. If you want to use the port for another purpose, you must delete the VCP setting using the request virtual-chassis vc-port command. You can run this command directly on the member whose uplink VCP setting you want to delete or through the primary member of the Virtual Chassis configuration.


Deleting a VCP in a Virtual Chassis configuration can cause the Virtual Chassis configuration to split. For more information, see Understanding Split and Merge in a Virtual Chassis.

If redundant VCP links of the same speed are connected between the same two member switches of a Virtual Chassis, the ports automatically form a VCP Link Aggregation Group (LAG) or bundle that distributes the inter-member VCP traffic load among them. See Understanding Virtual Chassis Port Link Aggregation for details.

Virtual Chassis Configuration

You configure and manage nearly all aspects of an EX Series or QFX Series Virtual Chassis through the primary switch of the Virtual Chassis. However, you can also configure Virtual Chassis parameters when a switch is a standalone switch not interconnected with other members yet, because any switch that supports being in a Virtual Chassis is by default a single-member Virtual Chassis with member ID 0. Upon connecting the switch with others in a Virtual Chassis, any Virtual Chassis configuration statements and uplink Virtual Chassis port (VCP) settings previously configured on the standalone switch remain part of its configuration.

You can set up an EX Series or QFX Series Virtual Chassis using a nonprovisioned or preprovisioned configuration. If you want to deterministically control the roles and member IDs assigned to the member switches when creating and managing a Virtual Chassis, use a preprovisioned configuration, which distinguishes member switches by associating their serial numbers with the member ID.

When adding new member switches to a preprovisioned Virtual Chassis, you might be able to simplify the procedure by using the autoprovisioning feature, which automatically converts the interconnecting links into VCPs when cabling the new switch into the Virtual Chassis under certain conditions and configuration settings. See Automatic Virtual Chassis Port (VCP) Conversion for details.

Configuring Interfaces for a Virtual Chassis

The member ID of an EX Series or QFX Series Virtual Chassis member switch functions as an FPC slot number. When you are configuring interfaces for a Virtual Chassis configuration, you specify the appropriate member ID as the slot element of the interface name.

The default factory settings for a Virtual Chassis configuration include FPC 0 as a member of the default VLAN because FPC 0 is configured as part of the ethernet-switching family. To include the FPC in the default VLAN, add the ethernet-switching family to the configurations for those interfaces.

Mixed and Non-mixed EX Series and QFX Series Virtual Chassis

A Virtual Chassis might consist of all the same type of switches or different types of switches in supported combinations.

Some combinations of switches in a Virtual Chassis comprise a mixed Virtual Chassis, which contains member switches that have operational differences requiring the Virtual Chassis to be configured with a mixed mode setting that enables all of the member switches to inter-operate successfully.

Some combinations of different types or models of switches can inter-operate in a Virtual Chassis without requiring the Virtual Chassis to be configured in mixed mode, such as different switches that can run the same Junos OS software image.

See Understanding Mixed EX Series and QFX Series Virtual Chassis for details on the different combinations of switches supported in a Virtual Chassis.

Virtual Chassis Member Switch Roles

Member switches in an EX Series or QFX Series Virtual Chassis operate in either a primary Routing Engine role, backup Routing Engine role, or linecard role. For some mixed Virtual Chassis, the member switches in the Routing Engine role are recommended (and in some cases required) to be particular types or models of switches. Any switch supported in a Virtual Chassis can operate in the linecard role.

A standalone switch that supports Virtual Chassis is by default a single-member Virtual Chassis that is assigned member ID 0 and operates in the primary Routing Engine role as the primary of itself. When connected and configured into a Virtual Chassis with other member switches, the switch will be assigned a unique member ID and might take on a different role.

A nonprovisioned Virtual Chassis uses a primary-role election algorithm to select the member switches that assume the primary and backup roles if an existing member switch in the Routing Engine role fails or when new member switches are added. In a preprovisioned Virtual Chassis, you assign the roles to each member switch when forming the Virtual Chassis and adding or replacing member switches.

See the following for details on EX Series and QFX Series Virtual Chassis member switch roles:

Global Management of Member Switches in a Virtual Chassis

The interconnected member switches in a Virtual Chassis operate and can be configured as a single network entity.

The serial console port and dedicated out-of-band management port on individual switches have global virtual counterparts when the switches are interconnected in a Virtual Chassis configuration. You can connect to the primary switch by connecting a terminal directly to the console port of any member switch. A virtual management Ethernet (VME) interface allows you to remotely manage the Virtual Chassis configuration by connecting to the out-of-band management port of any member switch through a single IP address. You can perform remote configuration and administration of all member switches of the Virtual Chassis configuration using the Junos CLI through the VME interface. See Understanding Global Management of a Virtual Chassis for details.

When setting up a Virtual Chassis on EX Series switches that support the EZSetup script, you can run EZSetup once to specify the identification parameters for the primary, and these parameters implicitly apply to all member switches of the Virtual Chassis.

On switches that support the J-Web user interface, you can view the Virtual Chassis as a single device in J-Web and apply various device management functions to all member switches of the Virtual Chassis.

High Availability Using Redundancy

Interconnecting EX Series or QFX Series switches into a Virtual Chassis increases your network’s high availability. A Virtual Chassis is more fault tolerant than a standalone switch because it can remain active, forward traffic and provide sub-second convergence in the case of a device or link failure.

Standalone switches that support only a single Routing Engine can operate with a primary and a backup Routing Engine when configured into a Virtual Chassis, and therefore support some high availability features that would otherwise not be available on the switch, such as Graceful Routing Engine Switchover (GRES) for hitless failover.

You can also increase fault tolerance within a Virtual Chassis by configuring other supported high availability features. For example, configuring Link Aggregation Group (LAG) bundles that include member links on different switches in the same Virtual Chassis enables traffic traversing the LAG to be redirected from a Virtual Chassis member switch that fails to links on another active Virtual Chassis member switch.

Adaptability as an Access Switch or Distribution Switch

A Virtual Chassis configuration supports a variety of user environments because it can be composed of different types of switches. You can select different switch models to support various functions. For example, you might set up one Virtual Chassis access switch configuration composed of full Power over Ethernet (PoE) models to support users sitting in cubicles equipped with PCs and Voice over IP (VoIP) phones. You could set up another Virtual Chassis configuration with partial PoE models to support the company's internal servers, and another to support the company's external servers. You can alternatively use a Virtual Chassis in a topology as a distribution switch.

Virtual Chassis Provisioning From the Factory-Default State Using the Phone-Home Client

Phone-home provisioning on a Virtual Chassis is a form of zero-touch provisioning (ZTP). With phone-home provisioning, when a device in the factory-default state boots up, a phone-home client (PHC) process automatically starts running on the device. The PHC gets bootstrapping information over the network from a central network management data source called the phone-home server (PHS), and installs the intended software image and configuration on the device without requiring any user interaction at the remote site.

The PHC also supports phone-home provisioning on some EX Series Virtual Chassis. Check Feature Explorer and search for phone-home to see the Virtual Chassis platforms that support phone-home provisioning. Phone-home provisioning on a Virtual Chassis is an extension of standalone device phone-home support.

See Provision a Virtual Chassis Using the Phone-Home Client for details about how the PHC works to provision a Virtual Chassis.

The PHS is usually part of a network management system (NMS) that supports phone-home provisioning. The network administrator enters the information that defines how the devices or Virtual Chassis at remote sites should be set up. An organization might have more than one PHS for redundancy.

Requirements for Phone-Home Provisioning to Work for a Virtual Chassis

The PHC only supports Virtual Chassis that meet the following conditions:

  • The Virtual Chassis member devices have dedicated or default-configured VCPs.

    Virtual Chassis Port Options explains the different VCP types and what’s supported on different platforms.

  • The Virtual Chassis members are all the same type of device.

    In other words, it isn’t a mixed-mode Virtual Chassis. See Understanding Mixed EX Series and QFX Series Virtual Chassis.

  • All member devices have the factory-default configuration.

    The PHC process only runs when a device or Virtual Chassis is in the factory-default state.

  • The Virtual Chassis member are interconnected in a ring topology using only dedicated or default-configured VCPs.

    How To Enable Phone-Home Provisioning on a Virtual Chassis shows a sample EX4300 Virtual Chassis that is wired this way.

  • At least one Virtual Chassis member has an active connection to the network and can access an available PHS that supports provisioning a Virtual Chassis.

    The connection to the PHS can be through the Virtual Chassis VME interface or any network-facing port on any Virtual Chassis member. See Understanding Global Management of a Virtual Chassis for more about how the VME interface works.