log_archive_format 12c not changing

We are on 12.1.0.2.0 and its a 4 node cluster and we have kept instances 2,3,4 down. Only instance 1 is working. I wanted to change the log_archive_format parameter to configure data guard manager but it is not changing.

SQL> show parameter format

NAME                                 TYPE        VALUE

------------------------------------ ----------- ------------------------------

log_archive_format                   string      %t_%s_%r.dbf

nls_date_format                      string

nls_time_format                      string

nls_time_tz_format                   string

nls_timestamp_format                 string

nls_timestamp_tz_format              string

permit_92_wrap_format                boolean     TRUE

star_transformation_enabled          string      FALSE

SQL> alter system set log_archive_format='%d_%t_%s_%r.dbf' scope=spfile sid='*';