A Cisco 5520 HA cluster (SSO) and a single Catalyst 9800 have a working mobility tunnel. After the primary 5520 fails, WLAN services remain but clients cannot roam between 5520 and 9800 without dropping. Which feature must be configured to fix roaming between controllers?

When a Cisco 5520 HA cluster operates in Stateful Switchover (SSO) mode, the active and standby controllers share the same mobility MAC address. This ensures that when a failover occurs, the mobility tunnel to the Catalyst 9800 WLC remains intact from the perspective of the 9800, as the mobility MAC address it communicates with does not change. Therefore, the Catalyst 9800 WLC must be configured with the mobility MAC address of the 5520 cluster, not its own mobility MAC, to properly maintain the mobility tunnel and allow seamless client roaming after a 5520 failover. Configuring "new mobility" on either controller is for enabling specific mobility features, not for addressing the fundamental issue of mobility tunnel identification during an HA failover.