Windows Server 2008 R2 networking : Planning and Deploying DHCP (part 1) - Planning for DHCP

Every device that communicates on a TCP/IP network must have an IP address. This includes computer workstations, laptops, network printers, routers, and servers. As you can imagine, the number of required IP addresses can add up. Think about managing a network with 5000 computers or even 10,000 computers. How do you assign IP addresses to each computer? This is where DHCP comes in. In this section, we will discuss what DHCP is and how it works. We will also cover installing and configuring DHCP and finish out the section learning how to troubleshoot DHCP.

1. Overview of DHCP

Most administrators know that managing a large network can be a daunting task at times. Can you imagine how daunting it would be to manually assign IP addresses to every device on your network? DHCP solves this problem by creating “pools” of IP addresses that can be “leased” by computers. DHCP is an industry standard protocol used to assign IP addresses to client computers. So exactly how does this process work? The DHCP process is outlined in the following steps and depicted in Figure 1.

Figure . DHCP IP Assignment Process.

1. A client configured to use DHCP for IP assignment sends a broadcast out to the network asking for an IP address.

2. The DHCP server picks up this broadcast and offers the requesting DHCP client an IP address from its pool.

3. The DHCP client sends a request back to the DHCP server basically stating that it truly wants to use the offered IP address.

4. The DHCP server then sends an acknowledgment back to the DHCP client stating that it has accepted the request.

5. On a scheduled basis, based upon a DHCP server setting, the client will renew its lease and send a request to the DHCP server for renewal.

6. If the DHCP server accepts the request, it sends another acknowledgment back to the DHCP client, informing it that it can continue to use the same IP address and resets the lease period. Once the new lease period expires, the client must perform steps 5 and 6 again.

DHCP provides not only IP addresses to clients, but also other configuration information, such as DNS Servers, the default gateway, and subnet mask information. Not only does DHCP prevent you from having to configure all of your devices, but also changes to your network can be made to all DHCP clients simply by making a configuration change on the DHCP server. The new configuration changes are pulled down by clients when they request a new IP address.

Best Practices Assigning IP addresses to servers In most cases, it is best practice to use static IP assignments for servers. Additionally, it is highly recommended that DHCP be never used to assign IP addresses to DNS Servers or AD domain controllers.

2. Planning for DHCP

Like DNS, DHCP is considered one of the most critical services on Windows network. If DHCP fails, then the client computers do not receive IP addresses and thus they cannot communicate on the IP network. If you want a reliable and highly available IP network, then DHCP failure is not an option. There are several factors to consider when designing your DHCP infrastructure.

• The number of physical and logical network locations requiring automatic IP configuration

• Router placement

• WAN connections and speed

• VLANs

• Availability requirements

• IP configuration options sent to clients

DHCP relay agents

A key point to remember is that DHCP requests are broadcasts that will not traverse most network routers. This means that you must put a DHCP server or DHCP relay agent on each IP segment or subnet. A DHCP relay agent is a component of Routing and Remote Access that simply forwards DHCP requests to another network segment (see Figure 2).

Figure 2. DHCP Relay Agent Configuration.

Notes From the Field DHCP forwarding on network routers Many of today’s network routers provide DHCP-forwarding services. If your network routers support DHCP forwarding, you may want to consider using them to forward DHCP requests instead of DHCP relay agents.

Planning for DHCP high availability

When deploying DHCP servers, you will want to ensure that you provide highly available DHCP services. There are a couple of ways to achieve this:

• Multiple DHCP servers —This is the most common method used to ensure DHCP availability. In this scenario, you can set up multiple DHCP servers and distribute the active IP addresses across them. For example, DHCP Server 1 might offer IP addresses from 10.1.1.1 to 10.1.1.50 and DHCP Server 2 might offer the IP addresses from 10.1.1.51 to 10.1.1.100. In the event that DHCP Server 1 fails, DHCP Server 2 would still be online and offer addresses to DHCP clients. When setting up multiple DHCP servers, you should consider how you want to split up your IP ranges. Several common practices exist, including the 80/20 split and the 50/50 split. The 80/20 split involves adding 80% of your IP addresses to one DHCP server and 20% to the other server. Using the 50/50 split, you place half of your IP addresses on one DHCP server and the other half on a second DHCP server.

• DHCP cluster —Using Windows Clustering Services you can set up a DHCP server on the top of a Windows Cluster. This active/passive availability option will allow a DHCP server to fail over to a secondary node in the cluster if the primary node fails.
  

You can use the Multiple DHCP server method, the DHCP Cluster method, or a combination of the two to provide high availability to DHCP.