DHCP client cannot detect IP address conflict

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A DHCP client sends gratuitous ARP packets to check whether addresses conflict after obtaining an IP address and before binding an IP address to the VLANIF interface. However, the VLANIF interface cannot go Up because the VLANIF interface has no IP address. As a result, the physical interface corresponding to the VLANIF interface cannot send gratuitous ARP packets with conflicting IP addresses to the CPU. The DHCP client cannot detect the conflicting IP addresses.
You can configure the DHCP server ping function on the DHCP server to check IP address conflicts. However, when there are many DHCP clients, the cost is high. You need to use it with caution through evaluation.

Other related questions:
DHCP clients on S series switch cannot detect IP address conflicts
Before a DHCP client binds an obtained IP address to a VLANIF interface, it proactively sends a gratuitous ARP packet to check for IP address conflict. The VLANIF interface does not have an IP address, so its protocol status cannot go Up. When the physical interface in the corresponding VLAN receives the gratuitous ARP packet with the conflicting IP address, it cannot send the packet to the CPU. As a result, the DHCP client cannot detect IP address conflict. To resolve this problem, configure the dhcp server ping function on the DHCP server to check for IP address conflict. When there is a large number of DHCP clients, this function requires a high cost, so you need to decide whether to use this function after thorough consideration.

How to resolve DHCP IP address conflict on S series switch
For S series switches (except S1700 switches), if IP addresses in a DHCP address pool have been configured for clients but have not been excluded from the address pool, the IP addresses may be allocated to other clients, resulting in IP address conflicts. For details about how to locate and rectify the IP address conflict problem, see "The IP Address Obtained by a Client Conflicts with the IP Address of Another Client" in Configuration Guide - IP Service.

Detect IP address conflicts on S series switch
On an S series switch, except S1700: When the IP addresses of two network devices conflict, the CPU usage on the switch becomes high and route flapping will occur on the switch. User services may be interrupted. To manage IP addresses efficiently and detect IP address conflicts promptly, enable IP address conflict detection. The switch can record logs for IP address conflicts. The configuration procedure is as follows: 1. Run the system-view command to enter the system view. 2. Run the arp ip-conflict-detect enable command to enable IP address conflict detection. By default, IP address conflict detection is disabled. 3. Run the display arp ip-conflict track command in any view to check IP address conflict records.

DHCP client cannot obtain a correct IP address
The request packets (DHCP discover) are broadcast by the DHCP clients. If multiple DHCP servers are located on the same subnet (for example, bogus DHCP server), the clients may obtain IP addresses from a bogus DHCP server. To address this problem, configure DHCP snooping. Then clients can receive DHCP packets from only the trusted DHCP server. Perform the following configuration: [Huawei] dhcp enable [Huawei] dhcp snooping enable [Huawei] interface gigabitethernet 1/0/1 //Enter the view of the interface connected to the DHCP client. [Huawei-GigabitEthernet1/0/1] dhcp snooping enable [Huawei-GigabitEthernet1/0/1] quit [Huawei] interface gigabitethernet 1/0/2 [Huawei-GigabitEthernet1/0/2] dhcp snooping trusted //Configure the interface of the DHCP server as the trusted interface. [Huawei-GigabitEthernet1/0/2] quit Note: - When you configure DHCP snooping on a Layer 2 access device, steps 1, 2, and 3 are mandatory and must be performed in the following sequence. - When you configure DHCP snooping on a DHCP relay agent, only steps 1 and 2 are required.

IP address obtained by a client on the AR conflict with the IP address of another client
The troubleshooting roadmap and solution are as follows: -Possible causes: The IP address is configured manually for the host, and is not excluded from the address pool when the DHCP server is deployed. -Troubleshooting: Disable the network adapter of the client or disconnect network cables and perform the ping operation on another host to check whether the host assigned with the IP address exists. After receiving a response, the IP address is configured manually. -Solution: a. Modify the manually configured IP address of the host. b. Exclude the conflicting IP address from the address pool on the DHCP server.

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