This article explains the error message "Upgrade to lower JSU spin number is not supported." on PTX evo platform.

The log messages show that the system attempted to install a Junos OS upgrade using the provided JSU image. However, the upgrade failed because the JSU image has a lower spin number than the currently running system software.

• Base Release: This represents the major version of the Junos OS, such as 23.4R2-S3. This signifies a significant release with major features and updates.
• Spin Number: This is a smaller increment within the base release. It typically indicates minor bug fixes, security patches, or small enhancements to the base release.

Installation failure messages:

root@re0> request system software add /var/tmp/junos-evo-install-ptx-x86-64-23.4R2-S3-J13.1-EVO.iso 
Adding software images. This process can take several minutes. Please be patient...
Download and Validate in Progress
re0: JSU base release 23.4R2-S3.11-EVO matches with Current Release: 23.4R2-S3.11-EVO.
re0: Error: Upgrade to lower JSU spin number 1 not supported. Current spin number 2.
re0: Installation failed
ERROR: Upgrade to lower JSU spin number is not supported. Node:re0 Image: re0:/var/tmp/junos-evo-install-ptx-x86-64-23.4R2-S3-J13.1-EVO.iso
Validation and download failed. Aborting upgrade.

This will happen due to the already installed JSU (J13.2) with a higher SPIN number on the router. The system won't allow the addition of the JSU with a lower SPIN number. 

root@re0> show version

Model: ptx10008
Junos: 23.4R2-S3.11-EVO
JUNOS Selective Update J13.1 [junos-evo-install-ptx-x86-64-23.4R2-S3-J13.2-EVO] <<<<<<<<<<<<<< 

When need to install JSU with a lower SPIN number then please remove the current JSU then install a JSU with a lower spin.

root@re0> request system software delete /var/tmp/junos-evo-install-ptx-x86-64-23.4R2-S3-J13.2-EVO.iso 

After this, you can install JSU (J13.1) without any issues.

root@re0> request system software add /var/tmp/junos-evo-install-ptx-x86-64-23.4R2-S3-J13.1-EVO.iso 
Adding software images. This process can take several minutes. Please be patient...
Download and Validate in Progress
re1: Running pre-checks for 'junos-evo-install-ptx-x86-64-23.4R2-S3-J13.1-EVO'
re1: Pre-checks pass successfully, copying files to software area
re1: Running post install commands...
re1: Post install sequence was successful.
re1: Boot version is now 'junos-evo-install-ptx-x86-64-23.4R2-S3-J8.1-EVO'
re0: Updating all nodes...
re0: Other nodes have been updated successfully
re0: Installation was successful
Image validation and installation succeeded.