logging codes

standy/active status logs already to /var/log/ltm, so if you are shipping your logs that message should be present. That said, you can also configure /etc/alertd/alert.conf to snmptrap or email from these events:

alert BIGIP_SOD_SODERR_SOD_STANDBY {
        snmptrap OID=".1.3.6.1.4.1.3375.2.4.0.14";
        lcdwarn description="Unit going standby." priority="5"
}
alert BIGIP_SOD_SODERR_SOD_ACTIVE {
        snmptrap OID=".1.3.6.1.4.1.3375.2.4.0.15";
        lcdwarn description="Unit going Active." priority="5"
}

Details are in Solution 3727

I believe it's the ltm log, do you see an entry in here when you fail over? If not then check your logging levels in the GUI (although I would guess the default level would catch this). Hope this helps, N.