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.

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