I am using Network Load Balancing on Windows server 2008. (Servers have two NIC's and the cluster is configured to run in unicast mode)

I can configure the cluster on either of the hosts and add the host on which I create the cluster. However, as soon as I try to add the other node (using NLB manager), I get the following:

  1. When I try to connect to the second host, an "Access denied" message appears.
  2. The logon dialog requests that I enter the username/password for an account on the host with Administrator priviledges.
  3. When I give valid administrator credentials, I get the message: "The RPC server is unavailable on the specified computer".

Turns out that the problem is simply UAC (User Account Control). The problem is not experienced if you:

  1. Use the Administrator account
  2. Turn off UAC and use an account in the Administrator group.

(I'm surprised that this is not well known, but I would guess that the Administrator account is used to configure NLB, so the situation rarely occurs)


Couple ways you can probably fix...

1) switch to multicast on the cluster

2) change your routing to explicitly use the other network adapter for the other host. It could be trying to use the first adapter...which would be verboten if in unicast mode.

In particular I'd look at:

1) registry setting MaskSourceMac for the NIC. This number will differ based on how you are connected (to switch, etc.)

2) Check your default gateway on the NICs. Depending on directions of your NLB (and what you're trying to do), once will often have the NLB NIC not have a default gateway, but the second NIC to have the default gateway.

The MS knowledgebase actually has a decent amount of information on NLB. Here's a couple links:


http://support.microsoft.com/?id=323431 (2003..i know...)

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.