Exchange Public Folder Best Practices: Understanding Referrals
This article describes best practices for deploying and configuring public folder client referrals in Microsoft® Exchange 2000 Server and Exchange Server 2003. This article assumes that you understand concepts about public folder referral. For a description of these concepts, see Chapter 7, "Managing Mailbox Stores and Public Folder Stores,” in the Exchange Server 2003 Administration Guide.
When a client tries to open an Exchange Server public folder, the Exchange server determines an appropriate folder replica for the client to access. This process is called referral. If a replica of the requested content exists on the Exchange server that serves the client request, the client accesses the local replica. If the replica does not exist on the local server, Exchange Server tries to find a replica in the same routing group.
When more than one replica exists in the local routing group, referral is based on a hash. This hash helps load-balance the referrals across the multiple replicas. You should also note that the hash is based on user properties that are typically static. A user almost always is referred to the same replica for that particular folder.
If no replica of the content exists in the routing group, the referral is determined by connector costs on the routing group, and the client is referred to the replica with the lowest cost.
Public folder referrals are transitive. For example, consider an organization that has three routing groups: A, B, and C. There is a routing group connector between A and B, and there is a routing group connector between B and C. In this scenario, clients in A can access public folder data in C, even though there is no explicit routing group connector between the two routing groups, A and C. This routing pattern occurs because public folder referrals are transitive in nature.
You should consider the transitive nature of public folder referrals when you plan your referral topology. In some scenarios, it may make sense to disallow public folder referrals over routing group connectors. If you disallow public folder referrals so that no replicas are accessible by clients, clients receive an error when they try to connect to the public folder. Therefore, make sure that the appropriate client groups can access public folders if you turn off referrals. You can turn off public folder referrals on the General tab of a specific connector's Properties page.
In Exchange Server 2003 Service Pack 1 (SP1), you can manually configure referrals. When you manually configure a referral, you can perform the following tasks:
Specify, at the server level, where all requests for public folder client referral are directed.
Specify multiple servers for referral.
Assign each server a cost to prioritize the servers in your referral list.
You can manually configure referral servers on the Public Folder Referrals tab on the Properties page for each server. This referral feature has been included in Exchange Server 2003 SP1 to help users troubleshoot public folder problems. This referral feature enables users to force client referrals from one server to another server regardless of the routing path. By using server-level referrals, you can perform the following tasks:
Control referrals across slow links.
Prevent referrals to new servers that have not replicated all content.
Prevent referrals to servers that are experiencing problems.
If the server in the custom list is not available, referral logic falls back to the routing referral process.
For more information about how to implement public folder referrals, see "Understanding Public Folder Referrals" in Chapter 7, "Managing Mailbox Stores and Public Folder Stores,” in the Exchange Server 2003 Administration Guide.
For More Information
To learn more about public folders in Exchange Server, see the following resources: