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Catalyst 9100 (無線AP)におけるPath MTU Discoveryの実行間隔の確認

Catalyst Wireless Catalyst 9800

無線APであるCatalyst 9100でのPath MTU Discovery (PMTUD)の実行間隔を確認した際の検証情報です。

検証時の環境は下記になります。

  • C9800-CL Version 17.15.1

  • CW9162I-Q Version 17.15.1.6

ドキュメントに記載が見つからなかったので、実機動作で確認を行った背景があります。

Path MTU Discoveryのデバッグ コマンド

Path MTU Discoveryの実行間隔はデバッグで確認を行いました。
デバッグは(WLC側ではなく)無線AP側で下記のコマンドを実行します。

debug capwap client pmtu
testap01# debug capwap client pmtu
Log rate is limited to 1500 logs/sec, more logs will be dropped if rate exceed the limitation.
testap01#

debug を停止する際は、個別にno debug capwap client pmtu で無効化するか、全てのデバッグを含めて undebug all で無効化します。

実機でのPath MTU Discoveryの間隔の確認

実機で確認した際は30秒間隔で実行されていました。

検証時のログを参考例として掲載します。実行間隔は下記の表の通りです。

実行間隔
03:49:03
03:49:33
03:50:03
03:50:33 
testap01# debug capwap client pmtu
Log rate is limited to 1500 logs/sec, more logs will be dropped if rate exceed the limitation.
testap01#
testap01#
testap01#[*03/20/2025 03:49:03.0099] PMTU: Timer Expired: Trying to send higher MTU packet, current PMTU 1485
[*03/20/2025 03:49:03.0099] PMTU: Timer: dcb->mtu 1485 dcb->attempt_mtu 1485 Sending Optimal Path MTU packet of size(next ladder) 1485
[*03/20/2025 03:49:03.0100] CAPWAP already has the same Path MTU(dcb->mtu = 1485, packetLen = 1485)
[*03/20/2025 03:49:03.0100] wtpEncodePathMTUPayload: Total Packet Size: 1485
[*03/20/2025 03:49:03.0100]
[*03/20/2025 03:49:03.0101] wtpEncodePathMTUPayload: Capwap Size is 1376.
[*03/20/2025 03:49:03.0101] [ENC]AP_PATH_MTU_PAYLOAD: pmtu 1485, len 1352, buffer len 1376
[*03/20/2025 03:49:03.0101] capwap_build_and_send_pmtu_packet: packet length = 1485 for current path MTU discovery
[*03/20/2025 03:49:03.0104] Ap Path MTU payload sent, length 1368
[*03/20/2025 03:49:03.0104]
[*03/20/2025 03:49:03.0104] WTP Event Request: AP Path MTU payload sent to 198.51.100.241, seq num 24
[*03/20/2025 03:49:03.0104]
[*03/20/2025 03:49:03.0129] WLC confirms PMTU 1485, updating MTU now. Length : 5 Update : 0
[*03/20/2025 03:49:03.0129] PMTU: Stopping the pmtu message timeout timer
[*03/20/2025 03:49:03.0129] PMTU: WTP already has the MTU 1485 and controller MTU : 1485. Update the click DP

testap01#
testap01#
testap01#[*03/20/2025 03:49:33.0235] PMTU: Timer Expired: Trying to send higher MTU packet, current PMTU 1485
[*03/20/2025 03:49:33.0235] PMTU: Timer: dcb->mtu 1485 dcb->attempt_mtu 1485 Sending Optimal Path MTU packet of size(next ladder) 1485
[*03/20/2025 03:49:33.0235] CAPWAP already has the same Path MTU(dcb->mtu = 1485, packetLen = 1485)
[*03/20/2025 03:49:33.0236] wtpEncodePathMTUPayload: Total Packet Size: 1485
[*03/20/2025 03:49:33.0236]
[*03/20/2025 03:49:33.0236] wtpEncodePathMTUPayload: Capwap Size is 1376.
[*03/20/2025 03:49:33.0237] [ENC]AP_PATH_MTU_PAYLOAD: pmtu 1485, len 1352, buffer len 1376
[*03/20/2025 03:49:33.0237] capwap_build_and_send_pmtu_packet: packet length = 1485 for current path MTU discovery
[*03/20/2025 03:49:33.0240] Ap Path MTU payload sent, length 1368
[*03/20/2025 03:49:33.0240]
[*03/20/2025 03:49:33.0240] WTP Event Request: AP Path MTU payload sent to 198.51.100.241, seq num 33
[*03/20/2025 03:49:33.0240]
[*03/20/2025 03:49:33.0258] WLC confirms PMTU 1485, updating MTU now. Length : 5 Update : 0
[*03/20/2025 03:49:33.0258] PMTU: Stopping the pmtu message timeout timer
[*03/20/2025 03:49:33.0258] PMTU: WTP already has the MTU 1485 and controller MTU : 1485. Update the click DP

testap01#
testap01#
testap01#[*03/20/2025 03:50:03.0367] PMTU: Timer Expired: Trying to send higher MTU packet, current PMTU 1485
[*03/20/2025 03:50:03.0368] PMTU: Timer: dcb->mtu 1485 dcb->attempt_mtu 1485 Sending Optimal Path MTU packet of size(next ladder) 1485
[*03/20/2025 03:50:03.0368] CAPWAP already has the same Path MTU(dcb->mtu = 1485, packetLen = 1485)
[*03/20/2025 03:50:03.0368] wtpEncodePathMTUPayload: Total Packet Size: 1485
[*03/20/2025 03:50:03.0369]
[*03/20/2025 03:50:03.0369] wtpEncodePathMTUPayload: Capwap Size is 1376.
[*03/20/2025 03:50:03.0369] [ENC]AP_PATH_MTU_PAYLOAD: pmtu 1485, len 1352, buffer len 1376
[*03/20/2025 03:50:03.0370] capwap_build_and_send_pmtu_packet: packet length = 1485 for current path MTU discovery
[*03/20/2025 03:50:03.0372] Ap Path MTU payload sent, length 1368
[*03/20/2025 03:50:03.0372]
[*03/20/2025 03:50:03.0372] WTP Event Request: AP Path MTU payload sent to 198.51.100.241, seq num 40
[*03/20/2025 03:50:03.0372]
[*03/20/2025 03:50:03.0398] WLC confirms PMTU 1485, updating MTU now. Length : 5 Update : 0
[*03/20/2025 03:50:03.0399] PMTU: Stopping the pmtu message timeout timer
[*03/20/2025 03:50:03.0399] PMTU: WTP already has the MTU 1485 and controller MTU : 1485. Update the click DP

testap01#
testap01#
testap01#[*03/20/2025 03:50:33.0502] PMTU: Timer Expired: Trying to send higher MTU packet, current PMTU 1485
[*03/20/2025 03:50:33.0503] PMTU: Timer: dcb->mtu 1485 dcb->attempt_mtu 1485 Sending Optimal Path MTU packet of size(next ladder) 1485
[*03/20/2025 03:50:33.0503] CAPWAP already has the same Path MTU(dcb->mtu = 1485, packetLen = 1485)
[*03/20/2025 03:50:33.0503] wtpEncodePathMTUPayload: Total Packet Size: 1485
[*03/20/2025 03:50:33.0504]
[*03/20/2025 03:50:33.0504] wtpEncodePathMTUPayload: Capwap Size is 1376.
[*03/20/2025 03:50:33.0504] [ENC]AP_PATH_MTU_PAYLOAD: pmtu 1485, len 1352, buffer len 1376
[*03/20/2025 03:50:33.0505] capwap_build_and_send_pmtu_packet: packet length = 1485 for current path MTU discovery
[*03/20/2025 03:50:33.0507] Ap Path MTU payload sent, length 1368
[*03/20/2025 03:50:33.0507]
[*03/20/2025 03:50:33.0508] WTP Event Request: AP Path MTU payload sent to 198.51.100.241, seq num 52
[*03/20/2025 03:50:33.0508]
[*03/20/2025 03:50:33.0535] WLC confirms PMTU 1485, updating MTU now. Length : 5 Update : 0
[*03/20/2025 03:50:33.0535] PMTU: Stopping the pmtu message timeout timer
[*03/20/2025 03:50:33.0535] PMTU: WTP already has the MTU 1485 and controller MTU : 1485. Update the click DP

testap01#

関連ドキュメント

debug capwap client pmtu に関してはCommand Referenceの debug capwap client の項目に記載があります。

英語

Cisco Catalyst 9100 Series Wi-Fi6/6E Access Point Command Reference, IOS-XE Releases - debug Commands [Cisco Catalyst 9100 Access Points] - Cisco
https://www.cisco.com/c/en/us/td/docs/wireless/access_point/ios-xe/command-reference/b-cisco-cat-ap-iosxe-cr/debug_commands.html#debug-capwap-client

日本語翻訳

Cisco Catalyst 9100 シリーズ Wi-Fi6/6E アクセスポイント（IOS-XE リリース）コマンドリファレンス - debug コマンド [Cisco Catalyst 9100 Access Points] - Cisco
https://www.cisco.com/c/ja_jp/td/docs/wireless/access_point/ios-xe/command-reference/b-cisco-cat-ap-iosxe-cr/debug_commands.html#debug-capwap-client