As with local continuous replication (LCR) and cluster continuous replication (CCR), standby continuous replication (SCR) in Exchange 2007 Service Pack 1 uses the concept of storage group copies. Because SCR introduces the ability to have multiple copies of your data, we use slightly different terms to describe the replication endpoints.
The starting point for a storage group that is enabled for SCR is called the SCR source. This can be any storage group, except a recovery storage group, on any of the following:
- Stand-alone Mailbox server
- Clustered mailbox server (CMS) in a single copy cluster (SCC)
- CMS in a CCR environment
The source must be running Exchange 2007 SP1. When using a standalone Mailbox server as the SCR source, you can also have LCR enabled for one or more storage groups, including storage groups enabled for SCR. You can have other roles (Client Access, Hub Transport, and/or Unified Messaging) installed, as well.
The endpoint for SCR is called the target, and the target can be either of the following:
- Stand-alone Mailbox server that does not have LCR enabled for any storage groups
- Passive node in a failover cluster where the Mailbox role is installed, but no CMS has been installed in the cluster
The target must also be running Exchange 2007 SP1. There are other requirements, as well. See Standby Continuous Replication for more information on SCR. In the case of both sources and targets, you can see the basic requirement for each: the Exchange 2007 SP1 Mailbox server role must be installed on both the source and target computers.
The last bullet for the SCR target is the reason for this blog post. There seems to be some confusion as to what we mean by a "Passive node in a failover cluster where the Mailbox role is installed, but no CMS has been installed in the cluster".
To help explain what we mean, let me describe how Exchange is installed into a failover cluster. You're probably familiar with the five server roles (Client Access, Hub Transport, Mailbox, Unified Messaging, and Edge Transport), but you might not realize there are two additional roles that can be installed, as well. These "roles" are not Exchange server roles, but rather CMS roles: specifically, the active clustered mailbox role and the passive clustered mailbox role.
The terms are used to tell Exchange Setup whether to install an active node or a passive node. For Exchange Setup, installing an active node means installing the Mailbox server role, and then installing a CMS. Installing a passive node means installing only the Mailbox server role. You do not create or install a CMS when you install the passive clustered mailbox role.
These roles are only expressed in the GUI version of Exchange Setup, so if you've installed your Exchange 2007 CMS' using only the command line version of Setup, you won't see these terms. In the command line, you'll simply see Mailbox server and Clustered Mailbox Server. It is the /newcms Setup option (and accompanying options) that dictate whether the active or passive clustered mailbox role is installed. If you include /newcms, the active clustered mailbox role is installed; if you do not use /newcms, the passive clustered mailbox role is installed.
When we say you can use a "Passive node in a failover cluster where the Mailbox role is installed, but no CMS has been installed in the cluster" we mean a Windows failover cluster in which one or more nodes exist, but only the passive clustered mailbox role is installed. You cannot have the active clustered mailbox role installed on any of the nodes in the failover cluster containing the SCR target(s). You can see a picture of what this looks like here.