cisco AIR-CAP3702I-D-K9 not able to join WLC 9800

[rsthakur]

*Aug 19 09:22:12.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.31.1.200 peer_port: 5246
*Aug 19 09:22:12.211: %DTLS-5-ALERT: Received FATAL : Certificate unknown alert from 172.31.1.200
*Aug 19 09:22:12.211: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.31.1.200:5246
*Aug 19 09:23:17.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.31.1.200 peer_port: 5246
*Aug 19 09:23:46.999: DTLS_CLIENT_ERROR: ../capwap/base_capwap/dtls/base_capwap_dtls_connection_db.c:2214 Max retransmission count reached for Connection 0xDF00738!

*Aug 19 09:24:16.999: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.31.1.200:5246
*Aug 19 09:24:27.000: %CAPWAP-5-DTLSREQSEND: DTLS connection request sent peer_ip: 172.31.1.200 peer_port: 5246
*Aug 19 09:24:27.211: %DTLS-5-ALERT: Received FATAL : Certificate unknown alert from 172.31.1.200
*Aug 19 09:24:27.211: %DTLS-5-SEND_ALERT: Send FATAL : Close notify Alert to 172.31.1.200:5246

[rsthakur]

Cisco Catalyst 9800-40 Wireless Controller working on 17.9.4 ISO

[Leo Laohoo]

FN63942 - Wireless Lightweight Access Points and WLAN Controllers Fail to Create CAPWAP Connections Due to Certificate Expiration

[rsthakur]

According to that, should I upgrade 17.9.4 to 17.9.5 ISO ?

Upgaredation of ISO will solve my problem ?

[Leo Laohoo]

Firmware upgrade will fix the issue. 

Upgrade to 17.12.4.

[Rich R]

@rsthakur Upgrading WLC IOS will not solve the problem by itself. 
As I already said below - you must follow all the steps in Field Notice 63942 to resolve the problem.

[marce1000]

 - . Have a checkup of your 9800-40 controller's configuration with the CLI command show tech wireless ; feed the output into : https://cway.cisco.com/wireless-config-analyzer/
                      Do nor use  a simple show tech as input for this procedure (use the full command as mentioned in green)

              Further debug AP joining issues with           https://logadvisor.cisco.com/logadvisor/wireless/9800/9800APJoin

 + Further what kind of links is between the APs and the controller , normal intranet or WAN (e.g.)

  M.

[Rich R]

Follow all the steps detailed in the field notice which Leo has linked to.
If you still see the problem do a factory default reset of the AP.