For each DB2 UDB instance (non-MPP) database partition, modify the file $DB2InstHome/sqllib/db2nodes.cfg. You need to create an entry for each database partition, and to assign the virtual IP address as the hostname. For example:

0 virtualhostname 0
1 virtualhostname 1

Note that the virtualhostname corresponds to the virtual IP address in the /etc/hosts file. Make sure that the virtual IP address is up and works.