EM 12c/13c Agent Unreachable (Agent is running but is currently not read to accept client requests)

EM 12c/13c create Agent Unreachable alerts as follows: 

Host=host01.dbaplus.ca 
Target type=Agent 
Target name=host01.dbaplus.ca:3872 
Categories=Availability 
Message=Agent Unreachable (REASON = The agent is running but is currently not ready to accept client requests). Host is reachable. 
Severity=Critical 
Event reported time=Nov 6, 2021 12:36:48 PM EDT 
Operating System=SunOS
Platform=sparc
Event Type=Target Availability 
Event name=Status 
Availability status=Agent Unreachable
Update Details:
Agent Unreachable (REASON = The agent is running but is currently not ready to accept client requests). Host is reachable.

It usually happens when the agent is monitoring many targets or heavily loaded.  Agent cannot respond to OMS heartbeat, it cause oms to generate Agent Unreachable alerts.

Check the value of agent property MaxInComingConnections with following command

${AGNET_INSTANCE_HOME}/bin/emctl getproperty agent -name MaxInComingConnections

Sample output if the property is configured,

$ emctl getproperty agent -name MaxInComingConnections
Oracle Enterprise Manager Cloud Control 13c Release 5  
Copyright (c) 1996, 2021 Oracle Corporation.  All rights reserved.
MaxInComingConnections=50

If the property is not configured, the output looks like following,

$ emctl getproperty agent -name MaxInComingConnections
Oracle Enterprise Manager Cloud Control 13c Release 5  
Copyright (c) 1996, 2021 Oracle Corporation.  All rights reserved.
MaxInComingConnections is not a valid configuration property

The property MaxInComingConnections can be configured by adding following line in agent configuration file "${AGENT_INSTANCE_HOME}/sysman/config/emd.properties",

MaxInComingConnections=200

If the property already exists, you can find this line (the value may be different depending on your system) in the file, and there is no need to add new line,  just update the value to a bigger number (e.g. 200).

Note, it is a best practice to do a backup before editing the configuration file.

After saving the change, restart the agent.