Dynamic Host Configuration Protocol

Superscopes

A superscope allows a DHCP server to provide leases from more than one scope to clients on a single physical network. Before you can create a superscope, you must use DHCP Manager to define all scopes to be included in the superscope. Scopes added to a superscope are called member scopes. Superscopes can resolve DHCP service issues in several different ways; these issues include situations in which:

Support is needed for DHCP clients on a single physical network segment—such as a single Ethernet LAN segment—where multiple logical IP networks are used. When more than one logical IP network is used on a physical network, these configurations are also known as multinets.
The available address pool for a currently active scope is nearly depleted and more computers need to be added to the physical network segment.
Clients need to be migrated to a new scope.
Support is needed for DHCP clients on the other side of BOOTP relay agents, where the network on the other side of the relay agent has multiple logical subnets on one physical network. For more information, see "Supporting BOOTP Clients" later in this chapter.

A standard network with one DHCP server on a single physical subnet is limited to leasing addresses to clients on the physical subnet. Figure 4.12 shows Subnet A before a superscope is implemented.

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Figure 4.12 DHCP Servers Using Single Scopes

To include the multinets on Subnet B in the range of addresses leased by the DHCP server shown in Figure 4.12, you can create a superscope that includes member Scopes 2 and 3 for Subnet B in addition to the scope for Subnet A.

Figure 4.13 shows the superscope configuration.

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Figure 4.13 DHCP Servers Using Superscopes

To include multinets on remote networks in the range of addresses leased by the DHCP server, you can configure a superscope to include member Scope 1, Scope 2, and Scope 3.

Figure 4.14 shows the scope configuration that includes the multinets on remote networks.

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Figure 4.14 DHCP Servers Using Superscopes for Remote Networks

Table 4.3 shows how two DHCP servers, both located on the same physical subnet, are each configured with a single scope.

Table 4.3 DHCP Scope for Servers A and B

DHCP Server Name	Starting IP Scope Address	Ending IP Scope Address
DHCP Server A	211.111.111.1	211.111.111.255
DHCP Server B	222.222.222.1	222.222.222.255

If DHCP Server A manages a different scope of addresses from DHCP Server B, and neither has any information about addresses managed by the other, a problem arises if a client previously registered with Server A, for example, releases its name during a proper shutdown and later reconnects to the network after a restart and tries to lease an address from Server B.

If Server B receives a DHCPRequest packet from the client to renew use of an address before Server A does, Server B (which does not contain any of Server A's IP addresses) rejects the request and sends a DHCPNak packet to the client. The client must then renegotiate a DHCP lease by broadcasting a DHCPDiscover packet onto the local subnet. Server B can send a DHCPOffer packet offering the client an address, which it can accept by returning a DHCPRequest for that address to Server B for approval, which occurs when Server B returns a DHCPAck packet to the client.

Nothing in this example prevents a client from having its request to renew an address rejected every time it connects to the network. In the process of rejecting and obtaining an address lease, the client might be offered an address that places it on a different subnet for which the client is not configured. By using superscopes on both DHCP servers, you can avoid both of these problems, and addresses are managed predictably and effectively.

Table 4.4 describes the same situation, but using superscopes. Both servers are located on the same physical subnet, and each is configured to allow multiple servers to provide addresses for a multinet.

Table 4.4 Superscope: DHCP Servers A and B

DHCP Server	Starting IP Scope Address	Ending IP Acope Address	Exclusions in the Scope
DHCP-ServerA	211.111.111.1	211.111.111.254
DHCP-ServerA	222.222.222.1	222.222.222.254	222.222.222.1 to 222.222.222.254
DHCP-ServerB	222.222.222.1	222.222.222.254
DHCP-ServerB	211.111.111.1	211.111.111.254	211.111.111.1 to 211.111.111.254

By configuring superscopes as described in this table, DHCP Servers A and B each recognize IP addresses assigned by the other. This prevents either server from negatively acknowledging attempts by DHCP clients to renew their IP address or to obtain an address from the same logical range of addresses. This works because DHCP Server B has knowledge of the scope in DHCP Server A via the superscope defined in DHCP Server B. Thus, if a DHCP client attempts to renew and its address belongs to one of the member scopes in DHCP Server B's superscope, Server B ignores the request.

Warning

When an IP address range that is too large for the subnet mask is specified, the administrator is given the option (by the DHCP Create Scope wizard) of creating a superscope. However, this might tax DHCP server resources. For example, if the new superscope includes more than 10,000 scopes, it might overload the server. In such cases, superscopes should be created manually with a smaller subset of scopes, or a smaller IP address range should be specified when using the wizard.