Causes for PCs obtain the same IP address through the S series switch

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The S series switch is used as the DHCP server to assign IP addresses to PCs:
1. If a DHCP client connects to a network again, the DHCP client broadcasts a DHCP Request packet carrying the IP address that the client has used. The used IP address is added in the Option50 field of packets. After receiving the DHCP Request message, the DHCP server checks whether the lease record exists based on the MAC address in the message. If so, the DHCP server replies with a DHCP ACK message 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:
Types of packets checked by S series switches with IPSG enabled
For S series switches (except S1700 switches), IPSG takes effect only for IP packets (except DHCP packets) but not for packets of other types such as ARP or PPPoE. With IPSG enabled, an S series switch checks only IPv4 packets in versions earlier than V200R001 and checks all IPv4 and IPv6 packets in V200R001 and later versions.

Causes for a long period of obtaining an IP address on an S series switch
For S series switches excluding the S1700, the main cause is that the STP function is enabled. (By default, STP on all switch interfaces is enabled.) When the interface changes from Down to Up, STP convergence is performed (the process takes about 30s). During STP convergence, a switch interface directly discards received packets. That is, DHCP Request messages are all discarded during the STP convergence period. As a result, IP addresses are obtained slowly in this case. Use either of the following method: - Run the stp edged-port enable command on the switch interface connected to a user terminal to configure the interface as an edge interface. If the switch is configured with the stp bpdu-protection command, the edge interface of the switch becomes Down when there are malicious attacks of BPDUs. To enable the edge interface to automatically go Up, run the error-down auto-recovery cause bpdu-protection interval interval-value command to configure the interface to restore Up state automatically. - If there is no physical loop, run the stp disable command to disable STP on the switch or interfaces connected to users. In another scenario where the switch functions as a DHCP server, a user obtains the IP address for more than 1 minute when the network cable of the PC is removed and reinstalled. This problem occurs due to a software bug. When the network cable of the PC is removed and reinstalled, the PC sends DHCP Request messages to request for an IP address. In earlier versions of switches, some problems occur in the software.

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.

Can clients connected to S series switch obtain IP addresses through DHCP
DHCP-enabled terminals that are connected to S series switches except S1700 switches can obtain IP addresses from DHCP servers when either of the following conditions is met: - The DHCP server function has been enabled on the switches. (By default, the DHCP server function is disabled on a switch.) - A DHCP server is available on the network and reachable to the terminals.

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