When enabling multiple wireless VLANs on the access point or bridge, multiple SSIDs are created with each SSID mapping to a default VLAN-ID on the wired side. However, as per 802.11 specifications, only one SSID can be broadcast in the beacons. The IT administrator defines a primary (Guest) SSID that is broadcast in the 802.11 beacon management frames. All other SSIDs are secondary SSIDs and are not broadcast in the 802.11 beacon management frames.
Refer to the following document:
http://www.cisco.com/en/US/products/hw/wireless/ps430/prod_technical_reference09186a00801444a1.html
