As explained in Chapter 1, MSMQ uses four types of servers to control message queuing:
You install one PEC on an enterprise network, on one PSC for each additional site.
One or more BSCs should be installed in each site that runs MSMQ-based applications that are critical to your businesses success. These BSCs can provide failure recovery if a PSC fails. BSCs also provide load balancing for MQIS queries, although MSMQ write requests must still be handled by the site controller (PSC or PEC).
The use of additional MSMQ routing servers (remember that the PEC, each PSC, and each BSC also functions as an MSMQ routing server) depends on the number of MSMQ dependent clients in your MSMQ enterprise and your use of session concentration. If your mobile independent clients don't use MSMQ-based applications while they are disconnected from the network, and you don't plan on using MSMQ session concentration, you probably don't need to install additional MSMQ routing servers.
Tip Installing additional BSCs and MSMQ routing servers is very easy, and does not require changes to your MSMQ enterprise settings (unless you are using session concentration). Because it is difficult to predetermine exactly how many BSCs and MSMQ routing servers your enterprise will require, start with a conservative number, but be prepared to add more as your message queuing load increases.
For best performance, do not install MSMQ servers on primary domain controllers (PDCs) or a backup domain controller (BDCs). PDCs and BDCs perform the resource-intensive tasks of maintaining and replicating the network accounts database and performing network login authentications.