"RPC Server Is Unavailable" Error Message When You Connect to NLB Cluster Host Through NLB Manager


When you attempt to connect to a Network Load Balancing (NLB) cluster host from another cluster host by using NLB Manager, you may receive the following error message:
The RPC server is unavailable
Connections through NLB Manager to an NLB cluster host from non-cluster hosts are successful. Only those connections initiated from other cluster hosts are unsuccessful.


This behavior occurs because cluster hosts contain only one network adapter, and cluster hosts function in unicast mode. This functionality is a limitation of the single-network-adapter unicast NLB configuration.

For additional information about these limitations, click the article number below to view the article in the Microsoft Knowledge Base:
197999 Single Network Interface Card Limitations with WLBS


To work around this issue, use any of the following methods:
  • Run NLB Manager from a non-NLB cluster host.
  • Enable multicast on the NLB cluster.
  • Add an additional network adapter for host-to-host communications.