這篇隨筆主要記錄的是Apple設備連接思科無線可能出現的問題，尤其是在思科WLC3504下部署的無線網絡，這種故障體現的尤為明顯。
For Single SSID
To support Apple iOS-based devices (iPhone/iPad) switching from one SSID to another on the same wireless access point, configure the Wireless LAN Controller (WLC) to enable the “FAST SSID change” function. This function helps ensure iOS-based devices are able to more quickly switch between SSIDs.
For Dual SSID BYOD
Fast SSID must be enabled to support dual SSID BYOD. When Fast SSID changing is enabled, the wireless controller allows clients to move faster between SSIDs. When fast SSID is enabled, the client entry is not cleared and the delay is not enforced. For more information about configuring Fast SSID on a Cisco WLC,
1、場景
WLC版本：8.5.131
WLC型號：AIR-CT3504-K9
2、示例
假設在該無線網絡下存在多個SSID，如果連接一個，再嘗試連接另一個，可能會出現“無法連接該網絡”/ "Unable to join the network SSID-name"
如下是一個例子，從RSCCIEW的SSID切換到TEST時出現的問題：
如果收集debug client xx.xx.xx.xx.xx.xx
可能會獲取到如下信息：
(WLC1) >*apfMsConnTask_0: Jun 05 13:56:04.571: 54:26:96:3e:4b:ee Association received from mobile on AP 00:22:bd:98:3a:30
*apfMsConnTask_0: Jun 05 13:56:04.572: 54:26:96:3e:4b:ee Deleting client immediately since WLAN has changed
*apfMsConnTask_0: Jun 05 13:56:04.572: 54:26:96:3e:4b:ee Scheduling deletion of Mobile Station: (callerId: 50) in 1 seconds
*apfMsConnTask_0: Jun 05 13:56:04.883: 54:26:96:3e:4b:ee Ignoring 802.11 assoc request from mobile pending deletion
3、解決方法
通過在WLC上開啟FAST SSID Change，該功能默認是關閉的。
When fast SSID changing is enabled, the controller allows clients to move between SSIDs. When the client sends a new association for a different SSID, the client entry in the controller connection table is cleared before the client is added to the new SSID. When fast SSID changing is disabled, the controller enforces a delay before clients are allowed to move to a new SSID.
當FAST SSID Change開啟是，控制器允許客戶端在SSID之間移動，這種情況下是當客戶端發送一個新的關聯信息到一個不同的SSID時，控制器上的客戶端連接條目在該客戶端連接到另一個SSID之前就會立即被清除掉。而當FAST SSID Change被關閉時，在客戶端移動到一個新的SSID之前WLC會讓這個操作延遲。
操作步驟：
3.1 登錄WLC的GUI，打開General界面
3.2 在Fast SSID Change下拉框選擇enable
3.3 點擊apply
3.4 點擊Save Configuration保存配置
通過命令行開啟：
(WLC1) >config network fast-ssid-change ? enable/disable] Enable or disables fast SSID changing for mobile stations (WLC1) >config network fast-ssid-change enable
(WLC1) >save config
Are you sure you want to save? (y/n) y Configuration Saved!
4、這個問題還可能伴隨一個問題，WPA+WPA2 參數的問題。
對於WPA和WPA2最好按照如下設置：
WPA+TKIP
WPA2+AES
最好不要混用。