need to upgrade f5 ltm from 11.4.1HF5 to 11.5.1 HF7

As there were changes to the default SSL cipher set between the specified TMOS versions I would closely monitor this aspect after running the upgrade. Especially if you are using your BIG-IP in a "closed environment" some clients may still rely on SSLv3 (now these ciphers are now disabled by default).

 

Heartbeat is compatible between both software versions. That´s why you can expect units to remain in active/standby even if they are temporarily on different software versions.

 

From my perspective it is best practice to bring a device in forced offline mode before doing the reboot for better control. (Be careful in case of vCMP deployments with the "forced offline" mode! All TMOS interfaces of the offline guest will become unavailable.) Monitor re-established device-group communication (tail -f /var/log/ltm) after the reboot before running the "release from offline".

 

Config sync will not work for units with different software versions.

 

I prefer to re-license the unit first and validate the service check date in license file (/config/bigip.license) before running the upgrade.

 

To avoid confusion about moving / copying configuration/license data I prefer installation to a "clean" installation volume.