Configuring standby OMS (OEM 11g)

Environment
Oracle Enterprise Manager 11.1
UNIX AIX 6.1

This is a simple configuration. There are two OMS hosts in different co-locations (host1, host2) and common DNS alias (oms).
There is no hardware SLB in this example.

Tips
When you configure OMS with SLB, DNS alias (oms.domain.com) should have different IP address from local hostname.

If they are the same, the result is:
Console Server Host : oms.domain.com
OMS is not configured with SLB or virtual hostname

If they are different, the result is:
Console Server Host : host1.domain.com
SLB or virtual hostname: oms.domain.com

After securing OMS with SLB alias, restart OMS (I didn’t see it in Oracle documentation), otherwise agents will not be able to register with the alias:
emctl secure oms -host oms.domain.com
emctl stop oms -all
emctl start oms

After configuring OMS as SLB/alias, you can point the alias oms.domain.com to host1.

Here is Oracle documentation how to Setup Standby Management Server.