If you have a 10g database in a 11.2g cluster and want to connect 10g database using SCAN then you should update local/remote listener parameters of 10g database manually. Here is an example for 2-node RAC:
mycluster-crs-scan.mydomain.com : My scan name (DNS)
mynode1-vip.mydomain.com: vip of node 1
MYSID1: Instance Name running on node 1
mynode2-vip.mydomain.com: vip of node 2
MYSID2: Instance Name running on node 2
alter system set remote_listener='mycluster-crs-scan.mydomain.com:1521' scope=both SID='*';
alter system set local_listener='(DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=mynode1-vip.mydomain.com)(PORT=1521))))' scope=both SID='MYSID1';
alter system set local_listener='(DESCRIPTION=(ADDRESS_LIST=(ADDRESS=(PROTOCOL=TCP)(HOST=mynode2-vip.mydomain.com)(PORT=1521))))' scope=both SID='MYSID2';
