Using Multiple DHCP Servers

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Using Multiple DHCP Servers

It is not recommended to have more than one DHCP server on a subnet because there is no way to control from which DHCP server a client receives an IP address lease. Any DHCP server that receives a client’s DHCP request broadcast can send a DHCP offer to that client. The client accepts the first lease offer it receives from a DHCP server.

If more than one subnet exists on a network, it is generally recommended to have a DHCP server on each subnet. However, if the DHCP relay agent or routers that support the forwarding of BOOTP braodcasts are used then request for DHCP addresses can be handled by a single DHCP server.

A DHCP server has an IP address scope configured for each subnet to which it sends DHCP offers. If the DHCP server receives a relayed DHCP request from a remote subnet, it offers an IP address lease from the scope for that subnet. To ensure that a DHCP client can receive an IP address lease even if a DHCP server is not functioning, you should configure an IP address scope for a given subnet on more than one DHCP server. Thus, if a DHCP client cannot obtain a lease from the local DHCP server, the DHCP relay agent or router passes the request to a DHCP server on a remote network that can offer a DHCP lease to the client.

For example, consider a network with two subnets, each with a DHCP server, joined by a RFC 1542-compliant router. For this scenario, Microsoft recommends that each DHCP server contain approximately 75 percent of the available IP addresses for the subnet the DHCP server is on, and 25 percent of the available IP addresses for the remote subnet. Most of the IP addresses available for a subnet can be obtained from the local DHCP server. If the local DHCP server is unavailable, the remote DHCP server can offer a lease from the smaller range of IP addresses available from the scope on the remote DHCP server.

If the range of IP addresses available are 120.50.7.10 through 120.50.7.110 for Subnet A and 120.50.8.10 through 120.50.8.110 for Subnet B, you could configure the scopes on each DHCP server as follows:

Subnet	DHCP Server A	DHCP Server B
	A	120.50.7.10	120.50.7.84	120.50.7.85 120.50.7.110
	B	120.50.8.10	120.50.8.34	120.50.8.35 120.50.8.110

You must ensure that no IP address is duplicated on another DHCP server. If two DHCP servers contain the same IP address, that IP address could potentially be leased to two DHCP clients at the same time. Therefore, IP address ranges must be split between multiple DHCP servers, as shown in the preceding example.

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