How to avoid going through Kerberos Authentication when username/passwd@tns is supplied?

Hi,

Actually we are using the following sqlnet.ora setup on both client and server side:

NAMES.DIRECTORY_PATH = (TNSNAMES)

SQLNET.KERBEROS5_CONF=/etc/krb5.conf

SQLNET.KERBEROS5_CONF_MIT=true

SQLNET.AUTHENTICATION_SERVICES=(NONE,BEQ,KERBEROS5PRE,KERBEROS5)

SQLNET.AUTHENTICATION_KERBEROS5_SERVICE=oracle

SQLNET.FALLBACK_AUTHENTICATION=TRUE

NAMES.DEFAULT_DOMAIN = L-BANK.INTERN

SQLNET.EXPIRE_TIME = 20

SQLNET.INBOUND_CONNECT_TIMEOUT=600

When connecting the following way and sqlnet trace level 16 is enabled

sqlplus user/password@tns

we see in the sqlnet trace that first Kerberos Authentication is tried, fails with ORA-12630 but continues with the

already opened session.

How  to avoid in this case Kerberos Authentication is tried?