Persistent SSID Broadcast
Access Points within a mesh network work as Root Access Points (RAP) or Mesh Access Points (MAP). RAPs have wired connection to the controller and MAPs have wireless connection to the controller . This feature is applicable only to the Cisco Aironet 1542 Access Points in the Flex+Bridge mode.
This feature is about the Root Access Points (RAPs) and Mesh Access Points (MAPs) broadcasting the SSID even when the WAN connectivity is down. This is required in order to isolate the responsibility; whether the fault is with backhaul or with the access wireless network, since there can be different operators owning each part of the network.
RAPs and MAPs broadcast SSID while in standalone mode, as long as the default gateway is reachable.
Also refer Mesh Deployment Guide for Cisco Catalyst 9800 Series Wireless Controllers.