How to achieve master key rotation in HADR setup?

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A key rotation on the primary will drive a key rotation automatically on the standby.

The key rotation on the standby is not instantaneous. The key rotation is written
on regular log records, and is not sent to the standby until a other log records are
sent.  So it’s possible and expected that you could do a key rotation on the primary,
and then immediately check for the key rotation on the standby and not see it.
Some write activity at the primary is required to drive sending the log records. There
is no explicit command to force this, although we can use archive log to pass the change at earliest.

i.e. using following command:

db2 archive log for db <dbname>

This will help to replay the Key rotation on HADR Standby.