Determining the Number and Placement of ISA Servers

ISA Server sizing concepts are not particularly complex, but often depend on the role that ISA will fill in an environment. In general, most organizations rarely tax the processor and memory utilization on an ISA server on updated server hardware, assuming that the server is used primarily for Internet-related traffic, such as web publishing or mail publishing rules. When an ISA server starts to be used for content caching, on the other hand, knowing the number of clients that will access the system becomes very important.

Sizing an ISA Server Deployment

Although there are no hard and fast guidelines for ISA Server 2004 sizing, some general suggested hardware minimums, shown in Table 4.2, should be followed when deploying an ISA server.

Table 4.2. Sizing ISA Server Hardware

	To 50 Users	50-500 Users	500+ Users
Processor	Single 550MHz Pentium III or equivalent	Single 1GHz Pentium III/IV or equivalen	Single 2GHz Pentium III/IV or equivalent (or greater)
Memory	256MB of memory	512MB of memory	1GB of memory (or greater)
Disk Space	150MB available (for installation of ISA software), plus space for logs and caching	150MB available (for installation of ISA software), plus space for logs and caching	8GB OS partition 10GB logs partition Additional 18GB+ partition for caching (if necessary)

NOTE

Table 4.2 lists only the minimal levels for a server with the indicated number of users. It is often wise to increase the capabilities of the server to avoid overtaxing its resources, particularly if it will act as a proxy server.

Choosing Between ISA Server Standard Edition and ISA Server Enterprise Edition

There is a fairly hefty difference in cost between the Standard version of ISA Server 2004 and the Enterprise version. It is therefore important to map out whether or not the Enterprise Edition is required. In general, Enterprise Edition deployments are required when any one of the following factors are true:

• Server failover and redundancy using network load balancing is required.

• Centralized logging to a SQL database for multiple ISA servers is required.

• Centralized firewall policy and/or array functionality is needed.

• Support for Cache Array Routing Protocol (CARP) web proxy traffic to optimize content caching for clients is required.

Deploying ISA to Branch Offices

The branch office deployment scenario is common for ISA configurations. In many cases, the ISA Server in the branch office may be the first line of defense for that particular office. In addition, site-to-site VPNs can be created easily between ISA servers in remote and parent sites, joining them together into a single contiguous network.

During the ISA design process, it is important to take into account the individual needs of each branch office and to examine whether a local install of ISA is relevant to that particular office.