Symptom
AP logs are showing the AP triggers MTU discovery several times per second:
As it mentioned in the MTU discovery documentation, the algorithm must be triggered every 30 seconds. Means that even if there’s a packet drop, the AP will change MTU in the next MTU discovery iteration.
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8550] WLC confirms PMTU 1485, updating MTU now.
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8550] PMTU: Stopping the pmtu message timeout timer
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8550] PMTU: Setting MTU to 1485, it was 1005
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] PMTU: Sending MTU update to WLC..
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] wtpEncodePathMTUPayload: Total Packet Size: 1485
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710]
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] [ENC]AP_PATH_MTU_PAYLOAD: pmtu 1485, len 5, buffer len 29
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] capwap_build_and_send_pmtu_packet: packet length = 1485 for current path MTU discovery
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] Ap Path MTU payload sent, length 21
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710]
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] WTP Event Request: AP Path MTU payload sent to 10.5.204.11, seq num 164
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710]
Jan 31 22:13:18 kernel: [*01/31/2023 22:13:18.8710] WLC confirms PMTU 576, updating MTU now
Captures are not showing any drop between the AP and WLC:
Conditions
AP model 9120
Image 17.3.6
Workaround
reboot the AP or restart the capwap
