今天1個AP出現注冊不上控制器的情況，隨手記一下。
日志提示 DTLS_CLIENT_ERROR: ../capwap/base_capwap/dtls/base_capwap_dtls_connection_db.c:224 Connection 0x306C3AC is already there for this server port 5246, Deleting it. Number of connections: 1
檢查AP的配置，證書時間等均是正確的，重啟AP問題依舊。
完整日志如下：
*Jun 1 06:53:52.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 06:53:52.511: %CAPWAP-5-DTLSREQSUCC: DTLS connection created sucessfully peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 06:53:52.511: %CAPWAP-5-SENDJOIN: sending Join Request to 172.17.10.10
*Jun 1 06:53:52.523: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.17.10.10:5246
*Jun 1 06:54:02.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 06:54:01.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 06:54:01.000: DTLS_CLIENT_ERROR: ../capwap/base_capwap/dtls/base_capwap_dtls_connection_db.c:224 Connection 0x306C3AC is already there for this server port 5246, Deleting it. Number of connections: 1
*Jun 1 06:54:01.000: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.17.10.10:5246
*Jun 1 06:54:01.011: DTLS_CLIENT_ERROR: ../capwap/base_capwap/dtls/base_capwap_dtls_handshake.c:929 Unexpected message received while expecting HelloVerifyRequest
*Jun 1 06:54:01.011: %DTLS-5-SEND_ALERT: Send FATAL : Unexpected message Alert to 172.17.10.105246
*Jun 1 06:54:01.011: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.17.10.10:5246

原因分析
由于該AP是Root AP，想到可能是mac地址沒有自動加入AP Policies，因此去搜索了一下，果然沒有添加這個mac，原因找到。
解決方法
添加AP的mac到AP Policies即可。

首先show version獲取AP的mac地址
32K bytes of flash-simulated non-volatile configuration memory.
Base ethernet MAC Address: D4:D7:48:6D:F0:E0
Part Number : 73-13538-01
PCA Assembly Number : 800-31224-01
PCA Revision Number : 03
PCB Serial Number : FOC15491L54
Top Assembly Part Number : 800-34851-02
Top Assembly Serial Number : FTX1542P367
Top Revision Number : A0
Product/Model Number : AIR-CAP1552E-A-K9
然后按照下圖添加mac地址，就可以了。

添加之后檢查該AP是否注冊上控制器。
在AP上看到日志如下，顯然已經注冊成功了。
*Jun 1 07:02:23.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 07:02:23.495: %CAPWAP-5-DTLSREQSUCC: DTLS connection created sucessfully peer_ip: 172.17.10.10 peer_port: 5246
*Jun 1 07:02:23.495: %CAPWAP-5-SENDJOIN: sending Join Request to 172.17.10.10
*Jun 1 07:02:24.395: %CAPWAP-5-JOINEDCONTROLLER: AP has joined controller WLC5508
*Jun 1 07:02:24.519: %LINK-6-UPDOWN: Interface Dot11Radio0, changed state to up
*Jun 1 07:02:24.863: %WIDS-6-ENABLED: IDS Signature is loaded and enabled
*Jun 1 07:02:25.239: %DOT11-4-NO_HT: Interface Dot11Radio0, Mcs rates disabled on vlan 2 due to not using AES encryption or encryption is not disabled
*Jun 1 07:02:25.239: %DOT11-4-NO_HT: Interface Dot11Radio0, Mcs rates disabled on vlan 3 due to not using AES encryption or encryption is not disabled
*Jun 1 07:02:25.275: %LINK-6-UPDOWN: Interface Dot11Radio1, changed state to down
*Jun 1 07:02:25.283: %LINK-5-CHANGED: Interface Dot11Radio1, changed state to reset
*Jun 1 07:02:26.263: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio0, changed state to up
*Jun 1 07:02:26.275: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio1, changed state to down
*Jun 1 07:02:26.303: %LINK-6-UPDOWN: Interface Dot11Radio1, changed state to up
*Jun 1 07:02:26.311: %LINK-6-UPDOWN: Interface Dot11Radio0, changed state to down
*Jun 1 07:02:26.319: %LINK-5-CHANGED: Interface Dot11Radio0, changed state to reset
*Jun 1 07:02:27.303: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio1, changed state to up
*Jun 1 07:02:27.311: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio0, changed state to down
*Jun 1 07:02:27.343: %LINK-6-UPDOWN: Interface Dot11Radio0, changed state to up
*Jun 1 07:02:28.343: %LINEPROTO-5-UPDOWN: Line protocol on Interface Dot11Radio0, changed state to up
*Jun 1 07:02:50.971: %CLEANAIR-6-STATE: Slot 0 enabled
*Jun 1 07:02:52.763: %CLEANAIR-6-STATE: Slot 1 enabled

控制器上再次檢查，可以看到，已經注冊成功了。