Microsoft KB Archive/265129

= Multihomed DHCP server does not allocate IP addresses =

Article ID: 265129

Article Last Modified on 10/30/2006

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APPLIES TO


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

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This article was previously published under Q265129



SYMPTOMS
When Dynamic Host Configuration Protocol (DHCP) is installed on a multihomed server, the server may not return IP addresses to eligible, requesting clients.



CAUSE
This behavior may occur if two or more network adaptors in the DHCP server are configured with an IP address in the same IP subnet, but only one of the adaptors is physically attached to the subnet where the clients requests are received. To view an example of this, refer to the &quot;More Information&quot; section of this article.



RESOLUTION
To resolve this issue, you can configure the network adapters for multihomed machines with IP addresses for different IP subnets and attach them to separate physical networks or VLANS.

In the &quot;More Information&quot; section below, if Adapter 2 were to be assigned the IP address of 172.16.0.2, with a subnet mask of 255.255.255.0, and attached to a separate subnet, then DHCP client/server communication would be restored as the IP address would be on a different logical IP network.

If two adapters are given IP addresses in the same IP subnet, ensure that network adapters are physically connected to the same subnet or virtual LAN (VLAN). This configuration is not recommended by Microsoft.



STATUS
The behavior of the DHCP server in determining an appropriate NIC to return DHCPOFFER packets to clients is by design.



Example
Adapter 1

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IP Address: 192.168.0.2

Subnet Mask: 255.255.255.0

Default Gateway: 192.168.0.1

Adapter 2

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IP Address: 192.168.0.3

Subnet Mask: 255.255.255.0

DHCP Server Configuration

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Scope: 192.168.0.1 - 192.168.0.254

Excluded Addresses: 192.168.0.2, 192.168.0.3

Lease Duration: 3 days

Adapter 1 is attached to the receiving subnet, and Adapter 2 is attached to a disjointed network. When a DHCP client requests a DHCP lease, it broadcasts a DHCPDISCOVER packet on the network, waiting for a response from an eligible DHCP server. In this example, Adapter 1 would receive the request, allocate the IP address of 192.168.0.4 from the applicable scope, and generate a DHCPOFFER packet to be returned to the client. However, because Adapter 1 and Adapter 2 have IP addresses on the same IP network, DHCP compares the IP addresses of the network adapters with the applicable scope of addresses, deeming either network adapter sufficient to return the DHCPOFFER packet to the client. If Adapter 1 is chosen by DHCP, the client receives the packet, as it is physically connected to the network. If Adapter 2 is chosen, the server times out when it attempts to send the DHCPOFFER packet to the client, and the lease fails to reach the client.

For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:

175767 Expected Behavior of Multiple Adapters on Same Network

Keywords: kbfasttrack kbtshoot kbenv kbnetwork kbprb KB265129

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