Hi,
1) Is it only once upgradable on controller? If FUS is too old and we need to upgrade it to the new one, in that case we might need 2 upgrades (check point X to Y then new version). What about this scenario?
You can upgrade twice. it's recommended to upgrade the same firmware version only once.it's advised to upgrade once the FUS because the WLC reboots and sometimes they will see that the FUS has not upgraded and upgrade again instead of waiting for 30 to 45 minutes.
2) How to determine FUS upgrade? Based on software version or Field Recovery image version?
HISTORY of : FUS 1.7.0.0 and 1.8.0.0 was highly recommended for 7.3.x software releases and above. (From my own experience, you should have already has those applied)
3) What is the minimum version required to upgrade to 1.9.0.0?
You can diretyl install version 1.9 for 5500 series.... and 2.0 for 2504 WLCs.
Regards
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