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

1. As grid user, use the command syntax mgmtua downgrade from 18c Grid home to downgrade Oracle Member Cluster where oldOracleHome is 12c Release 2 (12.2) Grid home and version is the five digit release number:

   $ $ORACLE_HOME/bin/mgmtua downgrade -member -oldOracleHome /u01/app/12.2.0/grid -toVersion version

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

   # crsctl start crs

   Note:

   You must start Oracle Clusterware on last downgraded node first, and then on other nodes.
6. 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"

7. As grid user, downgrade CHA models from any node where the Grid Infrastructure stack is running from 12c Release 2 (12.2) Grid home and Management Database and ochad are up:

   $ ./srvctl stop cha
   $ ./chactl import model -file /u01/app/12.2.0/grid/cha/model/os_gold.svm -name DEFAULT_CLUSTER
   $ ./chactl import model -file /u01/app/12.2.0/grid/cha/model/db_gold.svm -name DEFAULT_DB
   $ ./srvctl start cha

   In the example above, DEFAULT_CLUSTER and DEFAULT_DB are function names that you must pass as values.