Hi Experts
So we have two data center .I upgraded the HMC in one datacenter, very easy no problem, but in the second datacenter we have dual HMC connection. I am looking at the document from the ibm documentation. I found something that I am not comfortable following. which is this : Physically disconnect the ethernet cable from HMC #1 to the managed server(s)
o Upgrade HMC #1 to V7.7.7 and install recommended fixes
o Physically disconnect the ethernet cable from HMC #2 to the managed server(s)
o Reconnect the ethernet cable from HMC #1 to the managed server(s)
o Confirm HMC #1 connects successfully to managed server(s)
o Upgrade HMC #2 to V7.7.7 and install recommended fixes (start with step
o Reconnect the ethernet cable from HMC #2 to the managed server(s)
o Confirm HMC #2 connects successfully to managed server(s)
I want to ask has anyone done that before ? did you come across any challenges such as systems are not showing up in the HMC etc? if so how did you fixed it ? or is there anyway to avoid these steps ? if so then what are the consequences? Thanks in advanced .
DO IT!
There is a known problem, very rare, but nevertheless. If you have 2 different 7.x HMC versions in dual-HMC configuration and some specific firmware levels on your servers, you can loose the configuration of the managed system. If you are not so risky, do what IBM says 
And first do a backup of all your profiles.
known problem - taking it a bit far. rare - no, at well defined and documented points in time.
The danger is that there will be corruption of the representation of the data on the HMC's - as from time (well documented, don't ask me to recite when though (well, you can ask, but I cannot list them from memory 
)) changes to accommodate changes made in the firmware, or just to enhance HMC support of systems (e.g., the maximum of of partitions per HMC, or max number of partitions per managed system - one is only a change of the HMC, the other is a change of both!).
If dual HMC are connected AND at different levels - there is a chance that they corrupt each other and/or (worse) the managed system - because the data is formatted differently at the HMC level.
Hence the "mantra" - always update HMC first, then the firmware. An additional "play it safe" (i.e., even when documentation does not specifically mention it) with dual HMC is to:
- disconnect HMCA management ports so that it can not communicate with FSP ports (better FSP management network so these ports cannot communicate with HMCB either), and update HMCA (management continues via HMCB)
- disconnect HMCB from FSP (as HMCA had been done) - for this moment, there is no management of the systems (but they are up).
- Connect HMCA and give it time to update/sync itself with the systems
- If not successful, disconnect HMCA and reconnect HMCB - you can manage again - Call IBM (is what I would do now)
- EXPECTED: HMCA is now managing systems - so apply update to disconnected (with managed systems) HMCB, and when completed - connect and after sync dual HMC support is active again.
Hope this helps!