Use this procedure to downgrade Oracle Standalone Cluster to Oracle Grid Infrastructure 12c Release 2 (12.2) after a successful upgrade.

1. As grid user, delete the Oracle Grid Infrastructure 18c Management Database:

   $ $ORACLE_HOME/bin/dbca -silent -deleteDatabase -sourceDB -MGMTDB 

2. As root user, use the command syntax rootcrs.sh -downgrade from 18c Grid home to downgrade Oracle Grid Infrastructure on all nodes, in any sequence. For example:

   # $ORACLE_HOME/crs/install/rootcrs.sh -downgrade

   Run this command from a directory that has write permissions for the Oracle Grid Infrastructure installation user. You can run the downgrade script in parallel on all cluster nodes, but one.

3. As root user, downgrade the last node after you downgrade all other nodes:

   # $ORACLE_HOME/crs/install/rootcrs.sh -downgrade

4. As grid user, remove Oracle Grid Infrastructure 18c Grid home as the active Oracle Clusterware home:

   1. On any of the cluster member nodes where the rootupgrade.sh script has run successfully, log in as the Oracle Grid Infrastructure installation owner.

   2. Use the following command to start the installer, where /u01/app/18.0.0/grid is the location of the new (upgraded) Grid home:

      $ cd $ORACLE_HOME/oui/bin ./runInstaller -nowait -waitforcompletion -ignoreSysPrereqs -updateNodeList -silent CRS=false 
      ORACLE_HOME=/u01/app/18.0.0/grid "CLUSTER_NODES=node1,node2,node3" -doNotUpdateNodeList 

      Add the flag -cfs if the Grid home is a shared home.

5. As grid user, set Oracle Grid Infrastructure 12c Release 2 (12.2) Grid home as the active Oracle Clusterware home:

   1. On any of the cluster member nodes where the rootupgrade script has run successfully, log in as the Oracle Grid Infrastructure installation owner.

   2. Use the following command to start the installer, where the path you provide for ORACLE_HOME is the location of the home directory from the earlier Oracle Clusterware installation.

      $ cd $ORACLE_HOME/oui/bin 
      $ ./runInstaller -nowait -waitforcompletion -ignoreSysPrereqs 
      -updateNodeList -silent CRS=true 
      ORACLE_HOME=/u01/app/12.2.0/grid
      "CLUSTER_NODES=node1,node2,node3"

6. As root user, start the 12c Release 2 (12.2) Oracle Clusterware stack on all nodes.

   # crsctl start crs

7. As grid user, from any Oracle Grid Infrastructure 12c Release 2 (12.2) node, remove the MGMTDB resource as follows:

   $ $ORACLE_HOME/bin/srvctl remove mgmtdb -f

8. As grid user, run DBCA in the silent mode from the 12.2.0.1 Grid home and create the Management Database container database (CDB) as follows:

   $ $ORACLE_HOME/bin/dbca -silent -createDatabase -createAsContainerDatabase true 
   -templateName MGMTSeed_Database.dbc -sid -MGMTDB -gdbName _mgmtdb 
   -storageType ASM -diskGroupName ASM_DG_NAME
   -datafileJarLocation /u01/app/12.2.0/grid/assistants/dbca/templates
   -characterset AL32UTF8 -autoGeneratePasswords -skipUserTemplateCheck

9. Configure the Management Database by running the Configuration Assistant from the location $ORACLE_HOME/bin/mgmtca —local.
10. As grid user, use the post_gimr_ugdg.pl script from 18c Grid home, on the node where backup of model files exists in $ORACLE_BASE/crsdata/hostname/chad/projects/upgrade directory, where hostname is the name of the node:

    $ $ORACLE_HOME/bin/chactl post_gimr_ugdg.pl -downgrade -clusterType SC -destHome /u01/app/18.0.0/grid 
    -lowerVersion 12.2.0.1.0 -oraBase /u01/app/grid2
    

    Where:

    SC is the type of the cluster as Oracle Standalone Cluster. The value for -clusterType can be SC for Oracle Standalone Cluster, DSC for Oracle Domain Services Cluster, or MC for Oracle Member Cluster.

    /u01/app/18.0.0/grid is the Oracle home for Oracle Grid Infrastructure 18c.

    12.2.0.1.0 is the version of Oracle Grid Infrastructure to which you are downgrading.

    /u01/app/grid2 is the Oracle base for Oracle Grid Infrastructure 18c