This article explains the default behavior on EX2300, EX3400, EX4600, QFX5k series devices w.r.t xSTP BPDU (Bridge Protocol Data Unit) flooding, and provides a workaround to prevent it.

In the above-mentioned products, the default behavior of a switch is to address incoming xSTP BPDUs in the following way (when xSTP is not configured).

When a switch receives an incoming xSTP BPDU, it is sent to the PFE host path where it is re-inserted over the VLAN. This, in turn, may affect the link bandwidth as XSTP BPDU is flooded.

In order to prevent this from happening, no-mstp-flooding, no-vstp-flooding and no-stp-flooding have been introduced. By enabling these knobs under "switch-options", the packets are dropped at the PFE host path and never flooded/re-injected to the VLANs. 

CLI commands:

user@host# set switch-options no-mstp-flooding

user@host# set switch-options no-vstp-flooding

user@host# set switch-options no-stp-flooding

Notes:

1. Please verify the version on which you are trying to run the command. All three knobs are available for use in 15.1X53-D56 and 17.3 release onwards.  
2. no-mstp-flooding, no-vstp-flooding and no-stp-flooding are hidden knobs and do not auto-populate on pressing the Tab key or the Space bar. You will have to type the command completely.
3. This is not advised to be added to the configuration when xSTP is running on the setup since it prevents flooding of BPDUs to the VLANs.