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.
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.
a. Modify the manually configured IP address of the host.
b. Exclude the conflicting IP address from the address pool on the DHCP server.

Other related questions:
An AR used as a DHCP client cannot obtain an IP address
When the configuration is correct, you can shorten the IP address lease. If the IP address lease is long, after addresses in the address pool are allocated, the address that has been allocated to a client is no longer used and cannot be released. As a result, other DHCP clients cannot obtain the IP address.

Check IP addresses dynamically allocated to clients
On S series switches except S1700 switches, you can view IP addresses that have been dynamically assigned to users as follows: - If an interface address pool is used, run the display ip pool interface interface-pool-name used command. - If a global address pool is used, run the display ip pool name ip-pool-name used command. You can obtain the IP address used by each user according to the mapping between IP addresses and MAC addresses in the command output.

DHCP client cannot detect IP address conflict
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.

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.

Why different clients obtain the same IP address
When an S series switch functions as a DHCP server, a DHCP client accesses the network again in the following process: The DHCP client broadcasts a DHCP Request packet carrying the IP address allocated in the first time, that is, the Option 50 field in the packet. After receiving the DHCP Request packet, the DHCP server checks whether the lease record exists based on the MAC address in the packet. If so, the DHCP server replies with a DHCP ACK packet to notify the DHCP client that the requested IP address can be used. If not, the DHCP server waits for a new DHCP Discover message from the client.

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