https://www.cisco.com/c/en/us/td/docs/wireless/controller/8-1/Enterprise-Mobility-8-1-Design-Guide/Enterprise_Mobility_8-1_Deployment_Guide/WirelessNetwork_GuestAccessService.html
Anchor Controller Redundancy Priority
The guest anchor priority feature provides a mechanism that gives "active/standby" load distribution amongst the anchor WLCs. This is achieved by assigning a fixed priority to each anchor WLC, by distributing the load to highest priority WLC and in round-robin fashion if they have the same priority value.
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All guest clients are load balanced in round robin fashion amongst anchor WLCs.
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All guest clients are sent to anchor controller with highest priority in relation to local internal WLC.
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If an anchor fails, guest clients will be load balanced amongst remaining anchor WLCs.
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If an anchor fails, guest clients will be sent to the next highest priority or round robin if remaining anchors have same priority value.
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You can configure a priority to the guest anchor when you configure a WLAN. Priority values range from 1 (high) to 3 (low) or primary, secondary or tertiary and defined priority is displayed with guest anchor. Only one priority value is allowed per anchor WLC. Selection of guest anchor is round-robin based on a single priority value. If a guest anchor is down, the fallback would be on guest anchors with equal priority. If all guest anchors with same priority value are down, the selection would be on a round-robin basis on next highest priority and so on. Default priority value is 3. If WLC is upgraded to Release 8.1, it will be marked with priority 3. Priority configurations are retained across reboots. The priority configuration would be synchronized on HA pair for seamless switchover. Same set of rules apply in determining the anchor WLC regardless of IPv4 and/or IPv6 addressing. That is, highest priority value is determinant and not addressing including dual stack case.
