During discovery, one out of five fabric-enabled WLAN controllers is not being properly discovered. The firewall rules are the same for all the controllers. Which element is missing?

YANG is a data modeling language that describes configuration and state data carried over NETCONF, not an independent discovery or management protocol.

SNMP management enables polling and monitoring but is not used by Cisco DNA Center to discover or manage fabric-enabled WLCs in an SD-Access environment.

SNMP traps are unsolicited event notifications sent from a device to a manager and play no role in the initial device discovery process.

Cisco DNA Center uses NETCONF (TCP port 830) as the primary protocol for discovering and managing fabric-enabled Wireless LAN Controllers in an SD-Access deployment. If NETCONF is not enabled on one controller, DNA Center cannot establish a management session and the device will not appear as discovered. YANG data models are consumed over NETCONF but NETCONF itself must be enabled and reachable first.