In a distributed topology, most or all system components and messaging processes are distributed across multiple sites, usually at each remote site. Figure 12–2 shows a distributed topology.
You should consider a distributed topology for your site when:
Messaging at remote sites is mission critical.
Users send and receive large messages.
You have large user populations at remote sites.
Support personnel exists at remote sites.
There is poor bandwidth to remote sites.
If bandwidth significantly impacts your topology strategy, you should consider upgrading the bandwidth. In general, bandwidth is relatively inexpensive. You might also consider a Virtual Private Networking (VPN), which uses existing high bandwidth Internet pipes rather than dedicated lines behind a firewall.
There are advantages to implementing a distributed topology. Users at regional sites have faster access to their messages because they do not have to retrieve messages over the WAN. Furthermore, messages sent within a regional location will incur less messaging traffic than in a central topology. However, satellite offices still rely on the WAN. Therefore, if lots of message traffic is generated in a satellite office, the WAN might need to be upgraded.
The disadvantages of implementing a distributed topology are that typically you will have higher hardware costs and higher support costs as you maintain more hardware at more locations. Support costs are also higher because of the complexity of the distributed topology. For example, failover in a distributed topology is more difficult to implement than in a central topology. In addition, it is much slower to initially deploy Messaging Server because there are multiple servers spread across multiple sites.
Because Messaging Server accesses the LDAP directory, the LDAP server is a critical link in the mail delivery process. If you don’t use remote LDAP replicas, and the central LDAP is down, the messaging service will not be usable.