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In The Trenches: DHCP Clients Fail to Renew their IP Addresses

By ServerWatch Staff (Send Email)
Posted Sep 29, 2000


Thomas Shinder

Sometimes DHCP clients just don't get it. You turn them on and they don't renew their IP addressing information the way they should when their lease runs out. Or perhaps you're starting up a RIS client that needs to obtain its IP addressing information from a DHCP Server before starting the OS installation.

Sometimes DHCP clients just dont get it. You turn them on and they dont renew their IP addressing information the way they should when their lease runs out. Or perhaps youre starting up a RIS client that needs to obtain its IP addressing information from a DHCP Server before starting the OS installation.

When you run into this problem, you are likely check the usual suspects:

  1. Is there a network connectivity issue preventing the client from communicating with the server?

  2. Is the DHCP Server online?

  3. Does the DHCP Server have a default gateway defined? (an issue with the RIS client).

  4. Are there any IP addresses left in the scope?

But what do you do when the usual suspects fail to cough up the evidence?

The Spanning Tree Connection

The problem might be related to what type of network device the DHCP client is connected to. If the client is connected to a switch that performs a spanning tree calculation during port initialization, then the port the client is connected to may not be available for 10-15 seconds. This is long enough to prevent the DHCP client from renewing or obtaining IP addressing information, and thus preventing the TCP/IP stack from initializing.

This is not a problem on all switches that perform spanning tree calculations. Some devices will perform the calculations in the background, and thus the port becomes available immediately. Also, this type of functionality is only required on ports that are connected to other switching devices. Therefore you don't really need it enabled on ports that have clients directly connected to them.

The Fix

Microsoft recommends a few fixes for this problem:

  1. Turn off the spanning tree calculations on the ports directly connected to DHCP clients

  2. Assign static IP addresses to clients directly connected to the switch

  3. Place an unintelligent hub between the switch and the DHCP client(s).

Like many problems we encounter, the fixes are easy to implement. It's making the right diagnosis that makes the difference between ulcers and nirvana!

For More Information

For more information on how spanning tree calculations on the switch can affect your DHCP clients, check this article out at TechNet.

For deep details on DHCP in Windows 2000, check out our Managing Windows 2000 Network Services book.

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