Article ID: 268833
Article Last Modified on 2/28/2007
APPLIES TO
- Microsoft Windows 2000 Server
- Microsoft Windows 2000 Advanced Server
- Microsoft Windows NT Server 4.0 Standard Edition
- Microsoft Windows NT Server 4.0 Enterprise Edition
This article was previously published under Q268833
SYMPTOMS
If there are multiple Microsoft Dynamic Host Configuration Protocol (DHCP) servers in a multinetted environment, more than one DHCP server may have a lease for a given client. This can cause more lease usage than an administrator plans, and can potentially cause no leases to be available for some clients eventually.
This problem can occur under the following circumstances:
- The network is a single broadcast domain.
- The addressing scheme in use is multinetted (there is more than one IP subnet).
- There are multiple DHCP servers configured with scopes for different subnets.
CAUSE
When a client with a lease starts up, it broadcasts a DHCPREQUEST request without a server identifier. If a DHCP server receives the request and has no scope in the same subnet as that lease, it sends a DHCPNAK response to the client. The client enters a rebinding state and potentially obtains another lease from a different DHCP server in the broadcast domain. In this case, client leases are being held on both DHCP servers, although the client is using only one lease. For example:
- The client has a lease for address 10.1.1.15/24.
- The client is rebooted and broadcasts a DHCPREQUEST request for 10.1.1.15 with no server identifier.
- Other DHCP servers respond to the client's broadcast with a DHCPNAK response, except the server with a scope in the range of 10.1.1.1 - 10.1.1.254.
- The DHCP client enters a rebinding state.
- The client broadcasts a DHCPDISCOVER request and is offered several leases, the first lease being 10.2.2.15 from another DHCP server.
- The client broadcasts a DHCPREQUEST request with the server identifier of the offering server.
- The DHCP server broadcasts a DHCPACK response and the client uses this lease. Other DHCP servers stay silent at this time because the server identifier specifies another DHCP server.
- Later, the client is rebooted and gets a DHCPNAK response from other DHCP servers.
This same pattern could potentially continue with as many servers as are configured on the network.
RESOLUTION
To prevent the problem, configure each DHCP server in a multinetted environment with a scope within the range of each subnet that is being used for DHCP clients. Then, add each scope into a superscope on each DHCP server. This prevents the the servers from sending a DHCPNAK response to a client that is rebooted and broadcasts a DHCPREQUEST request.
The following sample configuration demonstrates how to resolve this problem in an existing multinetted environment:
DHCP server 1 IP address = 10.1.1.1/24
scope = 10.1.1.5 - 10.1.1.254
DHCP server 2 IP address = 10.2.2.1/254
scope = 10.2.2.5 - 10.2.2.254
On DHCP server 1, add the scope of 10.2.2.5 - 10.2.2.254, but exclude all of the addresses to prevent a duplicate address problem. The exclusion range is also 10.2.2.5 - 10.2.2.254. Then, add both of the scopes into a superscope. After you do so, server 1 does not respond with a DHCPNAK response to clients that are requesting addresses in the range of 10.2.2.5 - 10.2.2.254.
On DHCP server 2, add the scope of 10.1.1.5 - 10.1.1.254, and exclude the addresses of 10.1.1.5 - 10.1.1.254. Then, add both of the scopes into a superscope. After you do so, server 2 does not respond with a DHCPNAK response to clients that are requesting addresses in the range of 10.1.1.5 - 10.1.1.254.
MORE INFORMATION
For additional information about superscopes, click the article number below to view the article in the Microsoft Knowledge Base:
161571 Using DHCP 'Superscopes' to Serve Multiple Logical Subnets
Keywords: kbdhcp kbenv kbnetwork kbprb KB268833
