4. Stabilize Your Windows Infrastructure
A stable and resilient Windows infrastructure is a crucial element of any Exchange cluster deployment. For an Exchange 2003 cluster, all Active Directory (AD) domain controllers (DCs) and Global Catalog (GC) servers must run Windows 2003 or Win2K SP3 or later; the DCs and GC servers that support an Exchange 2000 cluster must run Win2K or later. Exchange 2003 and Exchange 2000 store configuration information in DCs and GC servers. Each DC holds a complete copy of all the objects in the DC's domain, plus a copy of objects replicated in the forestwide Configuration naming context (NC). A GC server holds a complete copy of all objects in the GC server's domain, plus partial copies of objects from all other domains in the forest. DSAccess is the Exchange component that locates and retrieves AD information from DCs and GC servers. From DCs, DSAccess retrieves information about Exchange entities such as administrative groups, connectors, Exchange system policies, and other servers in the Exchange organization. From GC servers, DSAccess retrieves user information such as email addresses and distribution group memberships. (For more information about DSAccess, see "Exchange 2000 SP2's DSAccess Component," July 2002, http://www.winnetmag.com, InstantDoc ID 25330.)
To ensure the stability of the infrastructure that your cluster relies on, you can build redundancy into your Windows 2003 or Win2K organization by implementing multiple GC servers, DNS servers, and WINS servers.
Multiple GC servers. Implement two GC servers in the same Windows 2003 or Win2K site and LAN in which your Exchange clusters reside. If DSAccess can't contact a GC server, the System Attendant will fail, causing a failover because the IS resource has a dependency on the System Attendant resource. Implementing two GC servers mitigates the effect of a GC server going offline. If another GC server is available in the site, Exchange will use DSAccess to locate that GC server. If no GC server is available in the site, Exchange will try to use GC servers in other sites, resulting in downtime as DSAccess attempts to locate a GC server. When DSAccess locates a GC server, the System Attendant and IS resources will come back online and service will be restored. Outlook clients also use GC servers to query and retrieve the Global Address List (GAL). If a GC server goes offline, Outlook sessions also are adversely affected. Deploying Outlook 2003 with cached mode enabled can reduce the impact and visibility to users when a GC server goes offline. When working offline in cached mode with no connection to an Exchange server or a GC server, Outlook uses the Offline Address Book (OAB) on the client to access directory information. Some deployments use separate sets of GC servers for additional resilience: Back-end GC servers support Exchange servers, and front-end GC servers provide directory information to Outlook clients.
Multiple DNS servers. Windows 2003 and Win2K use DNS to resolve server names to TCP/IP addresses and to locate resources. If a DNS server goes offline and no secondary DNS server is available, Exchange can't resolve server names to TCP/IP addresses and might experience DSAccess errors and nondelivery of mail.
Multiple WINS servers. Windows 2003 and Win2K use WINS servers to resolve NetBIOS names to IP addresses; Windows NT networks use WINS for name resolution. The Exchange Server 2003 Deployment Guide states that WINS is necessary for deploying Exchange 2003 or Exchange 2000; Exchange Setup and the ESM use WINS.
Step by Step
Planning, when combined with a stable and resilient Windows infrastructure, redundant clustering hardware, and specialized training for cluster administrators, is key to a successful Exchange cluster deployment. You can also improve your clusters by using proper configuration and adequate security measures, by knowing how to handle failovers, and by properly addressing whichever Exchange service pack your cluster uses. I'll discuss those steps in my next article.