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Virtual Chassis Fabric Overview


The Juniper Networks Virtual Chassis Fabric (VCF) provides a low-latency, high-performance fabric architecture that can be managed as a single device. VCF is an evolution of the Virtual Chassis feature, which enables you to interconnect multiple devices into a single logical device, inside of a fabric architecture. The VCF architecture is optimized to support small and medium-sized data centers that contain a mix of 1-Gbps, 10-Gbps, and 40-Gbps Ethernet interfaces.


A VCF is constructed using a spine-and-leaf architecture. In the spine-and-leaf architecture, each spine device is interconnected to each leaf device. A VCF supports up to twenty total devices, and up to four devices can be configured as spine devices. See Figure 1 for an illustration of the VCF spine-and-leaf architecture.

Figure 1: VCF Spine-and-Leaf Architecture
VCF Spine-and-Leaf

A VCF is based on either QFX5100 or QFX5110 switches as the spine devices, as follows:

  • You can configure a VCF with QFX5100 switches as the spine devices, referred to as a QFX5100 VCF. In an optimal QFX5100 VCF configuration, the leaf devices are also QFX5100 switches. You can, however, also create a mixed QFX5100 VCF by including any combination of QFX5100, QFX3600, QFX3500, and EX4300 switches as leaf devices.


    EX4300 multigigabit model (EX4300-48MP) switches are not supported in a VCF.

  • Starting in Junos OS Release 17.3R1, you can configure a QFX5110 VCF, which is a VCF with QFX5110-32Q switches as the spine devices. In an optimal QFX5110 VCF configuration, the leaf devices are also QFX5110 switches. You can, however, create a QFX5110 VCF by including any combination of QFX5110 and QFX5100 switches as leaf devices. A QFX5110 VCF that also contains QFX5100 devices is a non-mixed VCF because both types of switches run the same software image when interconnected into a VCF.

See Understanding Virtual Chassis Fabric Components for more information about the spine-and-leaf architecture, supported device types, and which device types are required or recommended to be in each role.

A VCF provides the following benefits:

  • Latency—VCF provides predictable low latency because it uses a fabric architecture that ensures each device is one or two hops away from every other device in the fabric. The weighted algorithm that makes traffic-forwarding decisions in a VCF is designed to avoid congestion and ensures low latency by intelligently forwarding traffic over all paths within the VCF to any destination device., ensuring predictable low latency for all traffic traversing the VCF.

  • Resiliency—The VCF architecture provides a resilient framework because traffic has multiple paths across the fabric. Traffic is, therefore, easily diverted within the fabric when a device or link fails.

  • Flexibility—You can easily expand the size of your VCF by adding devices to the fabric as your networking needs grow.

  • Investment protection—In environments that need to expand because the capabilities of a traditional QFX Series Virtual Chassis are maximized, a VCF is often a logical upgrade option because it enables the system to evolve without having to remove existing, previously purchased devices from the network.

  • Manageability—VCF provides multiple features that simplify configuration and management. VCF, for instance, has an autoprovisioning feature that enables you to plug and play devices into the fabric after minimal initial configuration. VCF leverages many of the existing configuration procedures from a Virtual Chassis, so that you can configure and maintain a VCF easily if you are already familiar with the procedures for configuring and maintaining a Virtual Chassis.

Release History Table
Starting in Junos OS Release 17.3R1, you can configure a QFX5110 VCF, which is a VCF with QFX5110-32Q switches as the spine devices.