How to locate when the AP keeps rebooting and DHCP address pool is exhausted

AP1--AccessSW1--CoreSW(dhcp server)--AccessSW2--AP2

AP1 reboot again and again in few minutes. 

In the dhcp server in CoreSW, DHCP address pool is exhausted. AP1 has occupied the majority of the DHCP addresses. And there are a lot of ARP with different ip of AP1.

AP1 reboot again and again in few minutes.   >    If the fit AP cannot obtain a DHCP address and cannot find an AC for registration, it will periodically reboot.

Why DHCP address pool is exhausted, and why there are a lot of ARP with different ip of AP1? we debug  arp all and dhcp all.

<H3C>dis debugging 

 ARP entry debugging switch is on 

 ARP packet debugging switch is on

 ARP error debugging switch is on 

 ARP active-ack debugging switch is on <H3C>sys 

*Dec 11 00:46:33:316 2023 H3C ARP/7/ARP_SEND: Sent an ARP message, operation: 1, sender MAC: 18c0-0977-55c0, sender IP: 172.26.22.163, target MAC: 0000-0000-0000, target IP: 172.26.22.163 

*Dec 11 00:46:33:326 2023 H3C ARP/7/ARP_ERROR: Packet discarded for ARP packet is not necessary to concerned.Interface: Vlan1 sender IP: 172.26.22.254 target IP : 172.26.22.163 

The ap send a arp of 172.26.22.163 but it will not reply the arp packet from gateway.

and we go ahead and debug the DHCP

<H3C>dis debugg

 DHCPv4 client packet debugging switch is on

 DHCPv4 client error debugging switch is on

 DHCPv4 client event debugging switch is on

<H3C>t m

The current terminal is enabled to display logs.

<H3C>t d

The current terminal is enabled to display debugging logs.

<H3C>sys

System View: return to User View with Ctrl+Z.

[H3C]*Dec 11 00:54:01:720 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: DHCP FSM state transfered (INIT-->SELECTING) successfully.

*Dec 11 00:54:01:720 2023 H3C DHCPC/7/PACKET:

To 255.255.255.255 port 67, interface Vlan-interface1

    Message type: REQUEST (1)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 1

    Client IP address: 0.0.0.0   Your IP address: 0.0.0.0

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPDISCOVER (1)

*Dec 11 00:54:01:720 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Successfully sent DHCPDISCOVER packet.

*Dec 11 00:54:02:255 2023 H3C DHCPC/7/PACKET:

From 172.26.22.254 port 67, interface Vlan-interface1

    Message type: REPLY (2)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 1

    Client IP address: 0.0.0.0   Your IP address: 172.26.22.229

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPOFFER (2)

*Dec 11 00:54:02:255 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: DHCP FSM state transfered (SELECTING-->REQUESTING) successfully.

*Dec 11 00:54:02:258 2023 H3C DHCPC/7/PACKET:

To 255.255.255.255 port 67, interface Vlan-interface1

    Message type: REQUEST (1)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 1

    Client IP address: 0.0.0.0   Your IP address: 0.0.0.0

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPREQUEST (3)

*Dec 11 00:54:02:259 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Successfully sent DHCPREQUEST packet.

*Dec 11 00:54:02:269 2023 H3C DHCPC/7/PACKET:

From 172.26.22.254 port 67, interface Vlan-interface1

    Message type: REPLY (2)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 1

    Client IP address: 0.0.0.0   Your IP address: 172.26.22.229

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPACK (5)

*Dec 11 00:54:02:270 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Beginning to detect IP address conflict via ARP.

*Dec 11 00:54:02:270 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Successfully sent ARP request for address 172.26.22.229.

*Dec 11 00:54:02:272 2023 H3C DHCPC/7/PACKET:

From 172.26.22.254 port 67, interface Vlan-interface1

    Message type: REPLY (2)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 1

    Client IP address: 0.0.0.0   Your IP address: 172.26.22.229

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPACK (5)

*Dec 11 00:54:02:272 2023 H3C DHCPC/7/ERROR:

 Vlan-interface1: Received a duplicate DHCPACK packet. Ignored the packet.

*Dec 11 00:54:02:273 2023 H3C DHCPC/7/ERROR:

 Vlan-interface1: Address conflicts.

*Dec 11 00:54:02:273 2023 H3C DHCPC/7/PACKET:

To 255.255.255.255 port 67, interface Vlan-interface1

    Message type: REQUEST (1)

    Hardware type: 1, Hardware address length: 6

    Hops: 0, Transaction ID: 2165756873

    Seconds: 0, Broadcast flag: 0

    Client IP address: 0.0.0.0   Your IP address: 0.0.0.0

    Server IP address: 0.0.0.0   Relay agent IP address: 0.0.0.0

    Client hardware address: 18c0-0977-55c0

    Server host name: not configured

    Boot file name: not configured

    DHCP message type: DHCPDECLINE (4)

*Dec 11 00:54:02:273 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Successfully sent DHCPDECLINE packet .

*Dec 11 00:54:02:325 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: Allocated IP (172.26.22.229) has been used by another host.

*Dec 11 00:54:02:325 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: DHCP FSM state transfered (REQUESTING-->INIT) successfully.

*Dec 11 00:54:02:325 2023 H3C DHCPC/7/EVENT:

 Vlan-interface1: DHCPDISCOVER will be sent in 4 seconds.

*Dec 11 00:54:06:720 2023 H3C DHCPC/7/EVENT:

It shows a clear step of dhcp and how this error happen:

Process:  dhcp discovery>dhcp offer>dhcp request>dhcp ack> arp detect(send by dhcp client, ap1) > AP found the ip assigned by dhcp is beeing used and sned a DHCPDECLINE to the dhcp server> a new dhcp cybel...

why ap thing 172.26.22.229 is being used? because the ap receive a arp from sw and Announcet for 172.26.22.229.

like this packet capture from the access sw port outbound direction:

The ap receive the arp Announcement which it sent out from the access sw. And dhcp server assign alot of ip address for this ap, but this ap will not use it.

root cause1: there are a loop in sw

root cause2: access port configure # port bridge enable#

solution: 1. solve the loop  in sw site or undo port bridge enable