Users may encounter scenarios where the Junos VM is running a different version and the host VM is running an older version. And they use the request system software add command with the non-vmhost image to upgrade instead of using the request vmhost software add command with a vmhost image on an MX device with a Next Generation Routing Engine (NG-RE).

If the upgrade version is not compatible with the current version of the vmhost and users try to perform the non-vmhost upgrade by using the request system software add command, users will get an error that states that the VMHost version is too old for Junos.

However, there is no risk involved if the Junos VM runs a different version and the host VM runs a different version.

Here's an example where the Junos OS versions are different.

labroot@jtac-mx240-r2016-re0> show version 
Hostname: jtac-mx240-r2016-re0
Model: mx240
Junos: 21.2X33.1     >>>>>>>>>>>>>> Junos Version

labroot@jtac-mx240-r2016-re0> show vmhost version 
Current root details,      Device sda, Label: jrootb_P, Partition: sda4
Current boot disk: Primary
Current root set: b
UEFI  Version: NGRE_v00.53.00.01 

Primary Disk, Upgrade Time: Thu Apr 13 10:04:24 IST 2023 

Version: set p
VMHost Version: 5.2217
VMHost Root: vmhost-x86_64-19.4R3-20200827_0454_builder
VMHost Core: vmhost-core-x86-64-19.4R3.11
kernel: 4.8.28-rt10-WR9.0.0.20_ovp
Junos Disk: junos-install-mx-x86-64-19.4R3.11 >>>>>>>>>>>>>> vmhost Version
 
Version: set b
VMHost Version: 5.2217
VMHost Root: vmhost-x86_64-19.4R3-20200827_0454_builder
VMHost Core: vmhost-core-x86-64-19.4R3.11
kernel: 4.8.28-rt10-WR9.0.0.20_ovp
Junos Disk: junos-install-mx-x86-64-19.4R3.11

When upgrading, the Junos VM software install checks for vmhost version compatibility with the host software at the time of upgrade. If the current vmhost version is compatible with the upgrade version of Junos, there will not be any error and the upgrade will go through smoothly only for the Junos VM and the host VM will continue to run the older Junos version without any issues.

However, if the upgrade version is not compatible with the current version of the vmhost and you try to perform the non-vmhost upgrade by using the request system software add command, you will get an error indicating that the VMHost version is too old for Junos as shown below.

labroot@jtac-mx240-r2016-re0> request system software add /var/tmp/junos-install-mx-x86-64-21.4X4.1.tgz no-validate 
Verified junos-install-mx-x86-64-21.4X4.1 signed by PackageProductionECP256_2023 method ECDSA256+SHA256
Checking vmhost version compatibility

VMHost version too old for Junos
ERROR: package junos-mx-x86-64-21.4X4.1 fails requirements
ERROR: Failed to add junos-install-mx-x86-64-21.4X4.1.tgz

The Junos VM software install checks for vmhost version compatibility with the host software at the time of upgrade. In this case, the 19.4R3.11 vmhost version is 5 and the 21.4X4.1 Junos VM software is not compatible with vmhost version 5. This is an invalid combination, and hence the upgrade will fail with the error as above.

However, there is no risk involved if the Junos VM runs a different version and the host VM runs a different version.

2023-05-17: Initial version