BDPU behavior change between the versions

The issue pertains to the BPDU, which was previously in the BLK state while running code version 20.4R3. After upgrading both QFX5110 devices to code version 23.4R3, the BPDU has transitioned to the FWD state.

QFX's connected to each other via inter-chassis connection and are connected to EX4300 on ae bundle.

Observed Behavior

• Junos Version 20.4R3: The STP behavior was such that the switch would block one of the ports to prevent network loops, resulting in the BPDU state being in BLK.
• Junos Version 23.4R3: After the upgrade, the BPDU state changed to FWD, indicating that the ports are now in the forwarding state.

Topology

qfx5110-1 (xe-0/0/6)=======ae46=======(xe-0/0/6) qfx5110-2
qfx5110-1 (xe-0/0/9) =======ae90=====ex4300
qfx5110-2 (xe-0/0/9) =======ae90=====ex4300



When the QFX's are in 20.4R3 version
 
root@qfx> show spanning-tree interface
 
Spanning tree interface parameters for VLAN 535
 
Interface                  Port ID    Designated         Designated         Port    State  Role
                                       port ID           bridge ID          Cost
xe-0/2/0                   128:490      128:490  33303.cce194cf3220         2000    FWD    DESG 
xe-0/2/1                   128:491      128:490  33303.cce194cf3220         2000    BLK    BKUP
 

When the QFX's are on 23.4R2-S3 version code:
 
root@qfx> show spanning-tree interface    
Spanning tree interface parameters for instance 0
Interface                  Port ID    Designated         Designated         Port    State  Role
                                       port ID           bridge ID          Cost
xe-0/2/0                   128:490      128:490  32768.cce194cf3220         2000    FWD    DESG 
xe-0/2/1                   128:491      128:491  32768.cce194cf3220         2000    FWD    DESG

Default VSTP Behavior in 20.4R3-S3.4: When VSTP (VLAN Spanning Tree Protocol) is not configured, the default behavior is to flood the packets.

Default VSTP Behavior in 23.4R2-S3.9: When VSTP is not configured, the default behavior is to drop the packets.

Solution:

To revert the VSTP behavior to flood the packets in the 23.4R2-S3 code, a configuration knob needs to be added:

1. Add the Knob: set switch-options vstp-flooding on both QFX devices. This will move the state to BLK.

show configuration switch-options |display set |match stp 

set switch-options vstp-flooding

> show spanning-tree interface 

Spanning tree interface parameters for VLAN 535

Interface         Port ID  Designated     Designated     Port  State Role

                    port ID      bridge ID     Cost

xe-0/2/0          128:490   128:490 33303.cce194cf3220     2000  FWD  DESG 

xe-0/2/1          128:491   128:490 33303.cce194cf3220     2000  BLK  BKUP