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For WLC HA ,can the HA controller act as redundant controller for more than one Active controller?
Short answer is - Yes you can but in an N+1 scenario as you describe the HA unit cannot perform SSO.
This link will help explain the possibilities and limitations.
what i finally got that , if we need SSO we just need HA Controller without license and the design will be N+N
but if we need to deploy N+1 scenario we will need Controller with license to support the APs which will fallback to it if the primary controllers are down , but in this case there will be no SSO