https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/8-
1/HA_SSO_DG/High_Availability_DG.html#pgfId-105076
After the WLCs are configured with Redundancy Management and Peer Redundancy Management IP Address and Redundant Units are configured, it is time to enable SSO. It is important to make sure that physical connections are up between both the controllers (that is, both the WLCs are connected back to back via Redundant Port using an Ethernet cable) and the uplink is also connected to the infrastructure switch and the gateway is reachable from both the WLCs before SSO is enabled.
Once SSO is enabled, it will reboot the WLCs. While it boots, the WLCs negotiate the HA role as per the configuration via Redundant Port. If the WLCs cannot reach each other via the Redundant Port or via the Redundant Management Interface, the WLC configured as Secondary may go in Maintenance Mode. Maintenance Mode is discussed later in this document.