Scope Configuration

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

By using the 80/20 split-scope configuration for fault tolerance and availability, scopes for all six subnets on the company network are defined on both DHCP servers. Exclusion ranges are used to allocate available addresses per scope, per server, as follows:

• The main office DHCP server is configured with 80 percent of the IP addresses available for lease to clients in each scope serving subnets A and B and 20 percent of the IP addresses available for lease to clients located at the branch office (subnets C through F).

• The branch office DHCP server has 80 percent of all addresses in all scopes available for lease to clients in the branch office (subnets C through F) and 20 percent of all addresses in all scopes available for lease to clients located in the main office (subnets A and B).

Thus, if either server suffers a hard-disk failure or other failure, the alternate server is available to assign and renew leases on all subnets.

To achieve the 80/20 rule, each Class C IP address range of 254 IP addresses available in each non-wireless scope is divided in the following manner:

• 20 IP addresses for static assignments.

• 187 IP addresses, or 80 percent of the addresses for lease, in the address pool of the DHCP server at the same site.

• 47 IP addresses, or 20 percent of the addresses for lease, in the address pool of the DHCP server at the other site.

The wireless scopes are divided in the following manner:

• 5 IP addresses for static assignments.

• 203 IP addresses, or 80 percent of the addresses for lease, in the address pool of the DHCP server at the same site.

• 51 IP addresses, or 20 percent of the addresses for lease, in the address pool of the DHCP server at the other site.

Table 2.4 shows the address pools and exclusion ranges configured on the main office DHCP server.

Table 2.4   Scope Configurations on the DHCP Server at the Main Office

Table 2.5 shows the address pools and exclusion ranges configured on the branch office DHCP server.

Table 2.5   Scope Configurations on the DHCP Server at the Branch Office

Lease Duration

Subnet B in the main office and Subnet C in the branch office are both wireless subnets. Because wireless clients (portable computers and other portable devices) are connected to and disconnected from the network in large numbers and for short intervals throughout the average day, lease duration on these two subnets is set for 15 minutes. These short lease times help to ensure that the maximum number of IP addresses are available in the scope as clients connect to the network.

Lease time for all other (nonwireless) subnets is eight days.

Scope Options

Each scope is configured with option 249, classless static routes. Using classless static routes, each DHCP client can be easily configured with the route to any destination on the network, and the subnet mask can be specified. Because each scope represents a physical subnet, the scope can be viewed as the start location for any message that is to be sent by a client to another subnet. The parameters used to configure option 249 are Destination, Mask, and Router. One or more static routes can be configured with option 249; the company provides all DHCP enabled clients on the network with routes to all other subnets using option 249.

This option is not configured as a server option because it maps routes between subnets, so no one set of values for the required parameters of Destination, Mask, and Router is always correct. For example, subnets A and D each use a router (that is, a VPN server configured with the Routing and Remote Access service and DHCP Relay Agent service enabled) to communicate with each other. Of course, the routers they use are different, and the destination is different in each case.

Server Options

The DHCP standard options shown in Table 2.6 are configured as server options at the main office DHCP server.

Table 2.6   Example DHCP Options

Other Settings

Scopes on both DHCP servers are configured with the same reservations, lease durations, scope options, and server options.