Essentially if you do not have any user mailboxes stores or public folder stores on an Exchange server that is operating as a front end server, then you can safely enable circular logging.  This will reduce the disk space requirements for the front end server. 

DO NOT enable circular logging if you have a single Exchange server environment or if your front end server hosts mailbox stores or public folder stores.  Logs are committed every time a full backup is performed on a store, thus eliminating the continual growth of logs on traditional back end server roles.

To enable circular logging

1. In Exchange System Manager, expand Servers, and then expand an Exchange server.
2. Right-click the storage group on which you want to enable circular logging, and select Properties. The storage group properties dialog box will appear.
3. Select the Enable circular logging check box, and then click OK to save the changes.
4. Exit Exchange System Manager and restart the Microsoft Exchange Information Store for the change to take effect.

You can read more than you ever wanted to know about circular logging at the following Microsoft Knowledge Base articles:

• http://support.microsoft.com/kb/258470
• http://support.microsoft.com/kb/147524
• http://support.microsoft.com/kb/296788
• http://support.microsoft.com/kb/314605