This article explains the behavior where setting the Cluster-ID to 0 does not enable the cluster.

When attempting to set the Cluster-ID to 0, the cluster is not enabled. The following output is observed:

root@SRX> set chassis cluster cluster-id 0 node 0 reboot
For cluster-ids greater than 15 and when deploying more than one
cluster in a single Layer 2 BROADCAST domain, it is mandatory that
fabric and control links are either connected back-to-back or
are connected on separate private VLANs. A CLI system reboot is
required for chassis cluster to be disabled

root@SRX>
*** FINAL System shutdown message from root@SRX ***
System going down IMMEDIATELY
----snippet----
FreeBSD/amd64 (SRX) (ttyu0)
login: root
Password:
--- JUNOS 24.2R2-S1.6 Kernel 64-bit JNPR-15.0-20250404.74caf3f_buil
root@SRX> show chassis cluster information
error: Chassis cluster is not enabled.

After the reboot, executing the show chassis cluster information command confirms that the chassis cluster is not enabled.

This is the expected behavior. Setting the Cluster-ID to 0 disables the chassis cluster.

For more information, refer to the following document: set chassis cluster cluster-id node node-number reboot