In CA APM cluster failover environment these problems are observed:

• Under the CEM Setup>Services>EM Configuration tab all Collectors show an "EM Running State" of "Unknown" instead of "Up". 
• Automatic Transaction Discovery (ATD) cannot be started and this message is received: "No matching EM collector found for <FQDN of EM running Tim Collection Service>"

1. When a Collector connects to the APM DB, it provides its FQDN, port number and a list of IP addresses for all bound Network Interface Cards (NICs) on which it is accepting incoming communications.
2. The Collector IP Address or hostname set in the MOM IntroscopeEnterpriseManager.properties file must match or be resolvable to the Collector FQDN in CEM. If there is no match the Collector status is set to "Unknown" instead of "Up"

The CA APM cluster failover environment had the following configuration:

• A Collector registers its FQDN and NIC IP Addresses with the APM DB and can be successfully used for assigning CEM Services.
• A MOM is using a Collector Virtual IP Address (VIP) and can successfully connect to it.

An "Unknown" running state will be reported instead of "Up" because the MOM is not able to map the Collector FQDN in CEM to the Collector VIP that it is configured with.
That also causes the Automatic Transaction Discovery (ATD) failure: "No matching EM collector found for <FQDN of EM running Tim Collection Service>".

APM 9.x, 10.x

Add a VIP <-> FQDN mapping for the Collector running the Tim Collection Service to the MOM hosts file (can also repeat for all Collector VIPs) and restart the cluster. The MOM will then be able to map the Collector FQDN in CEM to the Collector VIP that it is configured with.
The Collectors then had "EM Running State" of "Up" and the ATD startup error was resolved.

Compare the symptoms/impact of running states "Down" and "Unknown" on this documentation page:

Home > Troubleshooting > Enterprise Manager and Collectors Troubleshooting > Enterprise Manager Does Not Receive Data