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New Member

Upgrade backbone to cisco switches now I have DHCP issues

OK so we upgraded the network to be all Cisco devices with a 3508 at the core and 2950-48 switches for the desktops. THis replaced an old coax backbone with 10Mb hubs. Now that this has been done we seem to be having problems with machines getting conflicting IP addresses from the DHCP server. The DHCP was working fine for the 2 year prior that is has been setup and I was wondering if I need to configure something on the switches to help solve this. The switches are currently still running out of the box configs.

New Member

Re: Upgrade backbone to cisco switches now I have DHCP issues

You may be having a spanning tree problem. Many times, a client can issue a DHCP request while spanning tree convergence is taking place due a link state change on the edge port. This usually results in an apparent DHCP failure.

Try setting all of your edge ports for portfast and the problem should go away. The edge port mode must first be set to access. EX: Assume your are using fast ethernet ports 1 - 48 as edge ports for client node connections on your 2950-48.

Config terminal

Interface Range FA0/1 - 48

switchport mode access

spanning-tree portfast


write mem

The previous commands should get you fixed. ENSURE THERE IS NO POSSIBLE LOOPS THAT COULD BE FORMED BY BYPASSING NORMAL SPANNING TREE CONVERGENCE. That should not be a problem routinely on an edge port.

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