The following discusses Site Connector and remote procedure call (RPC) communications within sites over virtual private network (VPN) connections in Exchange Server. It covers the considerations and limitations of communication within sites and the use of site connectors over VPN connections.

Site Connector and Message Transfer Agent (MTA) communication within sites both use RPCs to connect, authenticate, and transfer e-mail messages. Microsoft does not recommend that you use RPCs over a VPN.
RPC communication requires a minimum bandwidth of 56 Kbps and does not tolerate latency on the network. As a general guideline, a Site Connector requires at least 56 Kbps of network bandwidth that is available exclusively for Exchange Server communications. It must be heavily stressed that the 56-Kbps transfer rate is a bare minimum for light mail flow with little or no directory replication traffic.

A VPN cannot provide guaranteed end-to-end bandwidth over a public network, such as the Internet. Although a VPN connection can be managed over a public network or over the Internet, the bandwidth is limited to what is currently available over the current router path. Periodically, latency can be expected on the network.

Note Site Connectors in a VPN over a public network are not supported. The options that are supported in a VPN over a public network are:

• X.400 Connector if you are running Exchange Server Enterprise Edition
• The Internet Mail Service with connected sites

Spanning an Exchange Server site across a VPN is also not supported. A supported configuration is to install or reinstall separate Exchange sites that are separated by a VPN. To maintain connectivity and reliable mail flow, Microsoft strongly recommends that you use an X.400 Connector if you are running Exchange Server Enterprise Edition or the Internet Mail Service if you are running Exchange Server Standard Edition with connected sites over a VPN.

If a Site Connector uses a VPN connection, you may have slow or intermittent mail flow, or it may stop mail flow completely. Event ID 9318, 9316, or 9322 messages may be logged by the Exchange Message Transfer Agent service in the Application event log.
These event ID messages are also logged if the Site Connector or the MTA communication within sites spans a VPN. These event ID messages may contain the following error codes :

• 1722: This error code indicates that a connection to the RPC server cannot be made. This may also indicate a possible name resolution problem. Verify your name resolution as a best effort approach to resolve this error.
• 1753: This error code indicates that the Endpoint mapper on port 135 is not responding to a port request.
• 5: This error indicates that access was denied. To resolve this issue, use the Override tabs on the Site Connector to verify the account and password.

233056 (http://support.microsoft.com/kb/233056/EN-US/) How to Replace a Site Connector with an X.400 Connector
• Comparison of X.400 and Site Connectors 196927 (http://support.microsoft.com/kb/196927/EN-US/) XIMS: How to Configure a Single Domain with Connected Sites via IMS 199326 (http://support.microsoft.com/kb/199326/EN-US/) XCON: Configuring X.400 Connectors to Communicate Through a Firewall For additional information about how to diagnose and test TCP/IP or NetBIOS network connections, click the following article number to view the article in the Microsoft Knowledge Base: 300986 (http://support.microsoft.com/kb/300986/EN-US/) HOW TO: Diagnose and Test TCP/IP or NetBIOS Network Connections
• How to Test Microsoft Remote Procedure Call Performance

The information in this article applies to

• Microsoft Exchange Server 5.5 Standard Edition