With clustering, only the mailbox server can exist in a failover cluster (Single Copy Clusters—SCC or Cluster Continuous Replication—CCR), with no other roles. Redundancy for the other roles is achieved through multiple servers and Network Load Balancing (NLB)/DNS round-robin.
There are five roles in Exchange 2007. The mailbox server role is the only clusterable role; therefore, if you use clustering for the mailbox server, none of the other roles can be installed on the same physical hardware because they don’t support clusters (unless you are using LCR, which can co-exist with other roles). The other redundancy options for the other roles are
* Client Access—You can use NLB or a third-party hardware-based NLB device for Client Access server high availability.
* Hub Transport—You can deploy multiple hub transport servers for internal transport high availability. Resiliency has been designed into the hub transport, as well as the Mail Submission Service on mailbox servers, for deployment of multiple hub transport servers.
* Edge Transport—You can deploy multiple edge transport servers and use round-robin DNS to load-balance activity across those servers.
* Unified Messaging—Unified messaging (UM) deployments can be made more resilient by deploying multiple UM servers where two or more are in a single dial plan. You can configure the Voice over IP (VoIP) gateways that UM supports to route calls to UM servers in a round-robin fashion. In addition, these gateways can retrieve the list of servers for a dial plan from DNS. In either case, the VoIP gateways will present a call to a UM server, and if the call isn’t accepted, the call will be presented to another, providing redundancy at the time the call is established.