How can I obtain an IP address when upgrading a vswitch?

Recently, the organization decided to upgrade the network device. All the originally used 3Com switches, and all the computers were assigned to two VLANs. Except some computers use fixed IP addresses, other computers obtain the address through a DHCP server. After the network is upgraded, the network center router remains unchanged. Most access-level switches are switched to Cisco Catayst 3550 or Cisco Catayst 2950, but the following situations occur after the upgrade: some computers cannot obtain the IP address when they are started or restarted, but can also obtain the IP address after manual setting.

Through observation, I found that this problem only occurs on Windows 98 computers. I think it may be because the computer is infected with a virus specifically targeting Windows 98, but no virus is detected with Norton or Kingsoft drug overlord, therefore, the cause of the virus is excluded.

Check the DHCP server again to check whether the DHCP server is running normally. After careful observation, I found that this problem only occurs on Windows 98 computers connected by Cisco switches, but no problem occurs on Windows 2000 or Windows XP computers, in addition, when using the 3Com Switch, Windows 98 computers can work normally.

I selected a Cisco Catayst3550-48 switch, carefully check its configuration, ARP table, Mac-Add Table and Log, no exception was found. I think that Windows 98 computers are not completely unable to obtain the IP address, but cannot be obtained at startup. This may be due to a problem during the initialization of the switch port.

The port of a Cisco switch is not immediately forwarded when it is connected to a computer. It has a range from Bloking (blocking), Listening (Listening), Learning (Learning) to Forwarding (Forwarding) the process of status transition. Therefore, it is very likely that there is a conflict between the Cisco switch and the network initialization of Windows 98 during this process.

If you change the port forwarding mode of a Cisco switch, that is, once the port is powered on, it will be in the forwarding status and the problem can be solved. So I added the configuration statement "spanning-tree portfast" in the port configuration of Cisco Catayst 3550 to solve this problem.

It seems that the network Startup Process of Windows 98 does conflict with the Spanning Tree Protocol of the Cisco switch, but it only appears when DHCP is used, however, the "spanning-tree portfast" command can only be used when the switch port is directly connected to a computer. If the port is cascade to another switch, this command will cause the spanning tree Protocol to run incorrectly. In this case, no other configuration is required, because the Spanning Tree Protocol is enabled on the Cisco switch by default, you can also add the "spanning-tree uplinkfast" command on the Cisco access layer switch port to reduce its convergence speed.

1. How to resolve conflicts between vswitch IP addresses and Server IP addresses
2. How to solve lan ip conflicts
3. How to solve computer IP conflicts in the LAN