To enable the Control-M/Server to correctly communicate with a Control-M/Agent of an older version, the correct Communication Protocol version must be configured for that Agent's version.  What are the correct Communication Protocol versions based on the version of the Agent?

Included below is the list of the Communication Protocol versions that correspond to each Control-M/Agent version:
 

Additional Information:

v9.0.21 supports only Protocol Version from 12 through 14.

After changing the Protocol version for the Control-M Agent you will need to recycle the Agent using shut-ag and start-ag commands for the changes to take effect.

Communication protocol 12 is used if the encryption method is changed on the agent.  This requires the Agent to be at 9.0.00.300 or higher
See http://documents.bmc.com/supportu/952/12/83/491283/491283.pdf#page=234 which explains the encryption type change and the protocol change to 12

NOTE: For HP-UX-ia64, Control-M supports protocol versions 7-11 only. Default: 11.

See http://documents.bmc.com/supportu/documents/95/12/509512/509512.pdf which provide further details.