Hello Kandy,
1) The ERS is implemented in a separate cluster group, which is always executed on a cluster node that does not contain the SCS group. In this case, the ERS host defines the potential backup node, also referred to as a failover target, for the SCS host.
2) The ERS is implemented as an independent system component, usually as a separate SAP instance. In this case, the ERS is triggering a polling process querying the cluster software in order to determine whether replication is required. This decision depends on how the ERS is executed on the current failover target node for the SCS instance.
Please check this :
http://help.sap.com/saphelp_nw70/helpdata/en/6d/e3a03cc24efd1de10000000a114084/frameset.htm
[Guide Deleted 19 May 2013]
Thank you,
Shyam
Removed broken links. Message was edited by: Jason Lax