This article explains how QFX5110 should be cabled in a VCF environment.
Virtual Chassis Fabric (VCF) is constructed using a spine-and-leaf architecture where each device in the fabric is either a spine device or a leaf device. In the spine-and-leaf architecture, each spine device is interconnected to each leaf device.
A VCF can have up to four spine devices, and up to twenty total devices. Each spine device has at least one direct Virtual Chassis port (VCP) connection to each leaf device in the VCF.
All traffic entering a leaf device can, therefore, be forwarded to any directly connected spine device and is always two hops away from any other leaf device—leaf device to leaf device traffic travels from the source leaf device to a spine device to the destination leaf device—within the VCF.
When adding redundant connections between the Spines, a loop may be introduced on the internal traffic and hence affecting services. Hence, the spine should not be connected to each other as it defies the purpose of spine-leaf architecture.
Please refer the following link for reference on cablinghttps://www.juniper.net/documentation/us/en/hardware/qfx5110/topics/topic-map/qfx5110-vc-vcf-connecting.html#cable-qfx5110-virtual-chassis-fabric