The requirements for FICON fabrics have always been different to Open-Systems (OS). The main cause for this is mainly that mainframe environments are deterministic as opposed to OS which use a discovery kind of methodology.
A few of the configuration settings in a cascaded FICON environment that have been mandatory settings were SCC in strict mode, a configured and active SCC policy and an IDID (Insistent Domain ID). The problem sometimes was that when these were not correctly configured the FICON channels could not be brought online in the first place or changes to these configurations really caused havoc resulting in IO errors and offline channels.
With FOS 7.3 Brocade introduced a new feature called HIF (High Integrity Fabrics) which basically means that before you can turn a channel online a lock has to be set on the above mentioned configuration options before you can vary-on the paths. z/OS will use a QSA (Query Security Attribute) ELS frame to determine if this feature is turned on and will fail if it is not. The HIF setting will lock the SCC configuration, IDID and SCC:S policy.
Upgrading with FMS mode enabled from Fabric OS v7.2.1 or prior to v7.3.0 or later requires that
the previous mentioned attributes be configured and active. If FMS mode is enabled on the switch and these are not configured, upgrade will fail. On a new switch with FOS 7.3 installed you first need to enable HIF before you can turn on FMS mode.
If for some reason you need to downgrade FOS to a version that does not support HIF you will first need to disable it. This also implies that you first need to disable FMS, then disable HIF and only then move to a lower FOS version. After that you can turn on FMS again but FOS will no longer protect you from making mistakes. 🙂
Kind regards, Erwin