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.

Other related questions:
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.

Long time to obtain IP addresses from S series switch
Users on S series switches except S1700 switches obtain IP addresses slowly because the STP function is enabled on interfaces of the switches. (By default, STP is enabled on all interfaces of a switch.) When an interface of a switch goes Up, STP convergence is performed for about 30 seconds. During STP convergence, the switch discards all received packets including DHCP request packets sent from users. As a result, the users obtain IP addresses slowly. You can resolve the problem using either of the following methods: - Run the stp edged-port enable command to configure the switch interface connected to users as an edge interface. If the stp bpdu-protection command has been configured on the switch, the edge interface of the switch goes Down when the switch receives attack BPDUs. To enable the edge interface to automatically go Up, run the error-down auto-recovery cause bpdu-protection interval interval-value command. - If no physical ring topology exists on the network, you can run the stp disable command on the switch globally or on the switch interface connected to users. Additionally, when the switch functions as a DHCP server and after you remove and reinstall the network cable, the PC obtains an IP address after 1 minute. The problem may be caused by a software bug. When you remove and reinstall the network cable, the PC sends DHCP Request messages to the DHCP server. In earlier versions of a switch, software cannot properly process this event.

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.

Why the clients on a DHCP snooping network cannot obtain IP addresses after they move
On a network using DHCP snooping-enabled S series switches, a binding table is generated on the switch after a user obtains an IP address. If the user is switched to another port of the switch without releasing the IP address, the user may fail to obtain this IP address and access the network. If a user attempts to apply for the same IP address with the same MAC address on a different port after the corresponding binding table has been generated, the switch cannot distinguish whether the user has switched to another port or an unauthorized user attempts to access the network. As a result, the switch does not modify the binding table, resulting in the user's failure to obtain the IP address. To solve this problem, you can delete the binding table from the switch.

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