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Preprovisioning a Virtual Chassis Fabric

Preprovisioning a Virtual Chassis Fabric (VCF) configuration allows you to assign the member ID and role for each device in the VCF. See Understanding Virtual Chassis Fabric Components and Understanding Virtual Chassis Fabric Configuration for details on the supported devices that can be interconnected into a non-mixed or mixed VCF.

Before you begin:

To preprovision a VCF:

  1. Make a list of the serial numbers of all the devices to be connected in the VCF. You can get a device’s serial number in the show virtual-chassis output or by following the instructions in Locating the Serial Number on a QFX5110 Device or Component, Locating the Serial Number on a QFX5100 Device or Component, Locating the Serial Number on a QFX3600 or QFX3600-I Device or Component, Locating the Serial Number on a QFX3500 Device or Component, or Locating the Serial Number on an EX4300 Switch or Component.

    Serial number values are case-sensitive.

  2. Decide the desired role (routing-engine or line-card) for each device.

    In a VCF, you configure two supported devices acting in the Routing Engine role into spine devices. All other devices—the spine devices not assuming the primary or backup Routing Engine role and all leaf devices—are configured into the linecard role as leaf devices.

  3. Set each device individually into fabric mode. If needed, also set the devices into mixed mode for a mixed VCF, and at the same time, request the device to reboot as part of the procedure to complete this step.

    If you are configuring a non-mixed VCF:

    If you are configuring a mixed VCF:


    A device whose fabric and mixed mode settings are improperly set will not properly join a mixed VCF. You can check the mode settings using the show virtual-chassis mode command.

    We recommend that you set the fabric and mixed mode before you interconnect your devices into a VCF to avoid the following issues:

    • Incurring downtime during VCF formation as the devices reboot to commit the fabric or mixed mode settings.

    • Manually correcting potential issues related to VCF formation because the device did not immediately join the VCF.

    You can, however, use the request virtual-chassis mode fabric local or request virtual-chassis mode mixed local commands to set a device into fabric or mixed mode after interconnecting your VCF.

    The fabric and mixed mode settings are automatically updated for a leaf device when it is interconnected into an autoprovisioned or preprovisioned VCF if the device is zeroized or has the factory default configuration. If the fabric or mixed mode settings are automatically changed when a leaf device is interconnected into a VCF, the leaf device automatically reboots in order to properly join the VCF. To avoid this potentially unexpected reboot and impact on VCF operation, as mentioned earlier, for best results, set the fabric and mixed modes and manually reboot each leaf device before cabling it into the VCF.

  4. After the reboot completes, log in to one of your spine devices that will be configured into the Routing Engine role.
  5. Specify the preprovisioned configuration mode:
  6. Associate a member ID with a serial number for each device in your VCF, and configure the role for each device:

    Configure two supported spine devices into the Routing Engine role, and the additional spine devices into the linecard role if your VCF supports three or more spine devices.

    Configure your leaf devices into the linecard role.

    For instance, if you wanted to preprovision a VCF with twenty member devices that uses member 0 and 1 in the Routing Engine role, members 2 and 3 as spine devices in the line card role, and the remaining devices as leaf devices:

  7. (Recommended) Configure a virtual management Ethernet (VME) interface for management of the VCF configuration:

    A VME accesses the device in the primary Routing Engine role using a management port, so cable management port em0 or em1 on each spine device in your VCF so the VME is available regardless of which spine device assumes the primary Routing Engine role. See Connecting a QFX Series Device to a Management Console.

  8. (Optional) The VCF forwards broadcast, unknown unicast, and multicast (BUM) traffic among the members of the VCF using multicast distribution trees (MDTs). By default, the VCF creates MDTs for every member of the VCF with that member as the root node of an MDT. If this default MDT creation method is not optimal for your installation, you can control which members become MDT root nodes.

    The set virtual-chassis member member-id fabric-tree-root configuration statement preempts the default method of creating MDTs, and specifies whether a member in a VCF can be an MDT root node. If this statement is configured for one or more members, MDTs are created only with the specified members as root nodes. See Understanding Traffic Flow Through a Virtual Chassis Fabric and fabric-tree-root for more on why you might want to choose this MDT creation method instead of the default method. Note that if you decide to use this option, we recommend that you specify all the spine members (and only spine members) as MDT root nodes.

    If desired, configure the spine devices in the VCF to be fabric MDT root nodes. For example, if you are preprovisioning a VCF with members 0 through 3 as spine devices (independent of the role):


    This option can also be configured anytime later during VCF operation if you observe internal VCF multicast traffic flow issues with default MDTs.

  9. Commit the configuration:
  10. (EX4300 switches in a mixed QFX5100 VCF using 40-Gbps QSFP+ interfaces as VCPs only) Perform one of the following tasks to convert the 40-Gbps QSFP+ interfaces into VCPs:

    Automatic VCP conversion only works when the interfaces on both ends of a link are not already configured as VCPs.

    This step is needed on EX4300 switches using 40-Gbps QSFP+ interfaces as VCPs because the 40-Gbps QSFP+ interfaces on EX4300 switches are configured as VCPs by default.

    If you are cabling the EX4300 switch to the spine switch with a DAC cable in the 40-Gbps QSFP+ interface, you must use the request virtual-chassis vc-port set command to manually convert the 40-Gbps QSFP+ interface on the spine device into a VCP, as described in the second bullet below.

    • Delete the VCP configuration on the 40-Gbps QSFP+ interface using the request virtual-chassis vc-port delete command.

      For instance, to delete the VCP configuration on all four QSFP+ interfaces on an EX4300-48T switch:

    • Manually configure the 40-Gbps QSFP+ interface on the spine device into a VCP using the request virtual-chassis vc-port set. For instance:

  11. Interconnect the spine device that you configured in the previous steps to all leaf devices by using supported interfaces that can be VCPs.

    In autoprovisioned or preprovisioned VCFs, the automatic VCP conversion feature is enabled and automatically configures SFP+ and QSFP+ interfaces into VCPs. You do not need to manually configure VCPs.

    If you want to configure an SFP+ or QSFP+ interface into a network interface, disable LLDP on that interface.

  12. Interconnect all other spine devices to all other leaf devices using supported VCP interfaces.
  13. Install the VCF feature licenses.

    For a VCF deployment, two license keys are recommended for redundancy—one for the device in the primary Routing Engine role and the other for the device in the backup Routing Engine role.

    To purchase a feature license for VCF, contact your Juniper Networks sales representative ( The Juniper sales representative will provide you with the feature license files and license keys. You will be asked to supply the chassis serial number of your switch; you can obtain the serial number by running the show virtual-chassis command.

    After obtaining the licenses, follow the instructions in Generating License Keys.