Network Load Balancing (NLB) clients cannot connect to the Windows Server 2008 NLB cluster by using the virtual IP address when NLB is running in multicast mode

Consider the following scenario:

• The Network Load Balancing (NLB) feature is enabled on a Windows Server 2008-based computer.
• NLB is configured to run in multicast mode.
• There is a router between the NLB cluster nodes and the client devices.
• The NLB cluster nodes have not recently communicated with any device on the client's subnet.

In this scenario, clients may experience intermittent connectivity failures when they try to communicate with a server that is part of this NLB cluster. Attempts to connect to the node-specific IP are successful. Outbound connection attempts from any NLB server nodes that use a node-specific IP are also successful.

When you try to respond to a client on a different subnet, the server must first send an ARP request for its default gateway. In multicast mode, this ARP request contains a combination of the unicast Virtual IP of the cluster and the multicast MAC address of the interface. Because some routers are designed or configured not to reply to packets with this mix of unicast and multicast, the server cannot discover the address of the default gateway. Therefore, the server cannot respond to the client's request.

Hotfix information

A supported hotfix is available from Microsoft. However, this hotfix is intended to correct only the problem that is described in this article. Apply this hotfix only to systems that are experiencing the problem described in this article. This hotfix might receive additional testing. Therefore, if you are not severely affected by this problem, we recommend that you wait for the next software update that contains this hotfix.

If the hotfix is available for download, there is a "Hotfix download available" section at the top of this Knowledge Base article. If this section does not appear, contact Microsoft Customer Service and Support to obtain the hotfix.

Note If additional issues occur or if any troubleshooting is required, you might have to create a separate service request. The usual support costs will apply to additional support questions and issues that do not qualify for this specific hotfix. For a complete list of Microsoft Customer Service and Support telephone numbers or to create a separate service request, visit the following Microsoft Web site: Note The "Hotfix download available" form displays the languages for which the hotfix is available. If you do not see your language, it is because a hotfix is not available for that language.

Important Windows Vista and Windows Server 2008 hotfixes are included in the same packages. However, only one of these products may be listed on the “Hotfix Request” page. To request the hotfix package that applies to both Windows Vista and Windows Server 2008, just select the product that is listed on the page.

Prerequisites

To apply this hotfix, the Network Load Balancing feature must be installed on the Windows Server 2008-based computer.

Restart requirement

You do not have to restart the computer after you apply this hotfix.

Hotfix replacement information

This hotfix does not replace a previously released hotfix.

File information

The English version of this hotfix has the file attributes (or later file attributes) that are listed in the following table. The dates and times for these files are listed in Coordinated Universal Time (UTC). When you view the file information, it is converted to local time. To find the difference between UTC and local time, use the Time Zone tab in the Date and Time item in Control Panel.

Windows Vista and Windows Server 2008 file information notes

The .manifest files and the .mum files that are installed in each environment are listed separately in the "Additional file information for Windows Server 2008 and for Windows Vista" section. These files and their associated .cat (security catalog) files are critical to maintaining the state of the updated component. The .cat files are signed with a Microsoft digital signature. The attributes of these security files are not listed.

For all supported x86-based versions of Windows Server 2008

For all supported x64-based versions of Windows Server 2008

For all supported IA-64-based versions of Windows Server 2008

To work around this problem, create a static ARP entry on each NLB node for the gateway. If you cannot do this, use some method for creating an outbound connection to a non-local device from the NLB node to populate the ARP cache. This method must support continuous communication between the devices to prevent the ARP cache entry from timing out.

Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.

The specific symptoms that you experience depend on the switch or the router and how it reacts to ARP when ARP is using both unicast IP and multicast MAC.

Additionally, a network sniff might be required to confirm the presence of the problem that is described in the "Cause" section. If the sender MAC address starts with "03-BF," this means that NLB is using the following multicast MAC address:

Additional file information for Windows Server 2008

Additional files for all supported x86-based versions of Windows Server 2008

Additional files for all supported IA-64-based versions of Windows Server 2008

Additional files for all supported x64-based versions of Windows Server 2008

For more information about software update terminology, click the following article number to view the article in the Microsoft Knowledge Base: