Do we need to install an agent to run the below command or can we use the job definition 'System_External_Command' to run it. Please advise.
brconnect -u / -c -f stats -t all
You would use SAP_ExternalProgramRun to run that, System_External_Command needs an agent.
Of course, the SAP system would need to have an ABAP stack, assuming you want to run it on an SAP system.
If you want to run it on an AS JAVA without agent, you would have to create a Java scheduler job, to schedule it through CPS/BPA, however, you would need an additional license for JXBP process server, see SAP NetWeaver Scheduler for Java
If you want to run it on the BPA/CPS server, that is another matter. You could try and run the program using Java ... it is not very reliable, though.