DHCP Design Principles

DHCP is heavily reliant on network topology, and is heavily relied upon by the hosts within a network. For DHCP to function at an optimal level, client computers must be able to access at least one DHCP server at all times. When developing a DHCP approach for your network, you must consider several things first:

• How many clients will be using DHCP for IP addresses?

• Where are these clients located and what roles do they have?

• What does the network topology look like?

• Are there any unstable WAN links that might cause a network outage if DHCP clients cannot contact a DHCP server for an IP address lease?

• Are there any clients that cannot use DHCP?

• Are there any clients that will be using BOOTP?

• Which IP addresses are dedicated and must be held outside the IP address pool?

• Will you be using Dynamic DNS?

DHCP clients do not wait for the DHCP lease to be over before beginning renewal. Instead, they begin the renewal at the point when 50 percent of the lease is up. For example, when a client has a ten-day lease, then after five days, the client sends the DHCP Request message to the DHCP server. If the server agrees to renew the lease, it responds with a DHCP Acknowledge message. If the client does not receive the DHCP Acknowledge response, the client waits for 50 percent of the remaining time (7.5 days after the original lease was made) before sending another DHCP Request message. This is repeated at 50 percent that remaining time (8.75 days after the original IP address lease). If the client cannot renew the address, or if the DHCP server sends a DHCP Not Acknowledged response, the client must begin a new lease process.

DHCP has only a couple of design requirements:
• You should have at least two DHCP servers to ensure redundancy. You can use clustering to ensure availability, but also keep in mind that two separate DHCP servers at different locations in the network can prevent DHCP problems resulting from a network link failure.

• You must either provide a DHCP server on each network segment or configure routers in between those segments to forward the DHCP messages.

When planning the DHCP servers, the network topology comes into play. It is critical you place DHCP servers at locations most available to the computers that need IP addresses.

Source of Information : Syngress The Best Damn Windows Server 2008 Book Period 2nd Edition

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