Though it is feasible to build a deployment around an instance of Directory Server installed on a single machine, the other Communications Suite components depend upon the directory as a core service to function. Thus, beyond trivial deployments, you should plan to deploy Directory Server in a redundant or highly availability configuration.
The first step toward making Directory Server more available is to establish a pair of master directory servers. Next, multimaster replication can be used to improve the LDAP write throughput and availability. If Sun Cluster is used for a high-availability deployment, then the two LDAP masters are clustered together. See Directory Server and High Availability for more information.