Microsoft KB Archive/168455

= DHCP Renewal Failures on Switched Networks =

Article ID: 168455

Article Last Modified on 2/20/2007

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APPLIES TO


 * Microsoft Windows 2000 Server
 * Microsoft Windows 2000 Advanced Server
 * Microsoft Windows NT Server 3.51
 * Microsoft Windows NT Server 4.0 Standard Edition

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This article was previously published under Q168455





SYMPTOMS
A client connected to an Ethernet switch may not receive a renewed DHCP IP address during startup. Releasing and renewing the IP address manually works.

Or, a client attempting use the preboot-execution (PXE) capabilities of a network adapter to install Windows 2000 Professional from a Windows 2000-based remote installation server may not be able to receive a DHCP or Bootp offer.



CAUSE
Some Ethernet switches have a feature that checks ports for a loop condition when ports become active (this is the learning mode for spanning tree algorithm). If a loop is found, traffic is not allowed across the port. This process of checking for a loop condition takes approximately 10- 15 seconds during which the Windows NT Workstation computer appears to be on the network, yet no traffic is being passed. Additionally, some newer switch features such as Port Trunking or Port Aggregation may increase the problem. The DHCPREQUEST packet (along with any other traffic during that time) is lost.

If you have a hub at the end of the port, the hub will have already brought the port up. You will only fail when connected directly to the switch.



RESOLUTION
To resolve this problem, disable the spanning tree algorithm feature of your Ethernet switch. This may not be necessary on all manufactures switches, some vendors may allow you to simply move spanning tree initialization to the background. This enables the switch port to almost immediately become functional, but still preform spanning tree discovery in the background (non-blocking). Please note that spanning tree is only necessary on ports that connect to other switches (or bridges). Additionally, if Port Trunking or Port Aggregation is enabled on ports attched to DHCP clients, then either disable these features or convert the client to a static IP address if these features are required. Ports that have only clients connected do not need spanning tree enabled. For information on how to do this, please contact the switch manufacturer.

Additional query words: prodnt renew catalyst

Keywords: kbnetwork kbprb KB168455

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