I discussed about OID replication concepts like supplier, consumer , replication agreement , ASR (Database Advanced Server Replication) vs LDAP based replication here and here

You can see Mahendra’s post on OID Multi-Master replication here and steps to configure OID 11g LDAP based multi-master replication here

Steps to configure OID in multi-master replication with screenshots are covered in My Oracle Support Note 1052278.1 How to Setup 11g OID Multimaster Replication.
In this post I am going to cover key points that supplement note 1052278.1 How to Setup 11g OID Multimaster Replication

Why and when to implement OID multi-master replication ?
OID uses Oracle database as its persistent store both for configuration and user data. For high availability (as shown on left side of above image) in one data center, you can configure two or more OID nodes connected to RAC Database (RAC database provides high availability for OID database). You can configure two OID application tier instances connected to same RAC database using steps in EDG here.

For OID high availability across sites (two data centers situated miles apart) or in situation where you don’t have RAC database license,  You can achieve high availability across sites using OID Multi-Master replication. In OID multi-master replication, OID is always available (Active-Active) on both Primary and Standby site. Change on OID@standby are replicated to OID@Primary and vice versa and two OID’s remain in sync.

How OIDs configured in Multi-Master remain in sync ?
You can configure two or more OID’s in multi-master replication either using LDAP based replication (LDAP commands using change log are used to keep to OID stores in sync) or ASR based replication (database advanced replication feature is used to sync two OID stores)

Note1: LDAP based replication is preferred replication
Note2: ASR based replication is mandatory if you are using 10g SSO (OSSO)

Things good to know for OID LDAP based multi-master replication
1) If you have two OID application nodes (node1 & node2) connected to same OID database (RAC_DB1) and two another OID application nodes (node3 & node4) connected to second OID database (RAC_DB2) as shown in figure above, then to configure multi master replication between a) node1/2 with RACDB1 and b) node3/4 with RACDB2
a) configure/start OID replication daemon OIDREPLD only on one of two nodes from OID1 or OID2
and
b) configure/start OID replication daemon OIDREPLD only on one of two nodes from OID3 or OID04

2) You can configure OID Multi-master replication using Replication Wizard (using Enterprise Manager) or command line tool (remtool)
Note: Replication Wizard (via enterprise manager) is recommended approach to configure OID multi-master replication 

3) You can configure replication on SSL or Non SSL. If you wish to configure replication on SSL then use LDAP SSL port between members of replication nodes

4) Bootstrap is the process to initially loading existing data (including OID schema extension) from one OID to another OID member in replication . You can bootstrap by selecting checkbox as shown below during configuration.

Note: Do not select bootstrap for both the replication members

More on OID multi master replication attributes and replication daemons in next post