6 Tips to Keep in Mind When Configuring DHCP Servers

In today’s corporate networks, DHCP services can be utilized to eliminate time-consuming, error-prone manual IP assignments. As a result, the IT administrator’s day-to-day job becomes much easier. The primary goal of an administrator is to ensure that a DHCP server is properly deployed, maintained, and managed for good performance and availability.

When deploying a DHCP solution, administrators must consider several key factors. For example, it’s important to know the number of DHCP servers required to cater to their organizations IP needs. Further, if a centralized or decentralized implementation should be utilized. Additional components to consider include: server redundancy options, server hardware and storage requirements, bandwidth availability, the growing size of the network, and preferred vendors.

6 Microsoft DHCP service tips that ensure your network is performing at an optimal level

  1. Configuring Exclusions: DHCP servers can automatically hand out IP addresses, however organizations tend to use static IPs for devices like servers, printers, etc. The problem with this is that static IPs sometimes fall within the range of a DHCP. Such IPs, including that of the DHCP server can be configured as exclusions and not just left under manual management. In addition, exclusions can be configured for a single IP or a range of IP addresses. 
  2. Specifying Lease Durations: Setting long lease durations may lead to non-availability of IP addresses. As organizations expand, mobile devices are increasingly being used. However, the time spent on the network by users is rather brief.  In turn, they tend to leave the network without releasing the IP address. Conversely, having too short a lease period ends up generating large amounts of renewal traffic. So, remember to carefully weigh current and expected IP needs of your network. For example, providing 20% more IP addresses than are expected to be in use and setting the lease time to 1 day may be useful to cater to mobile users. But, for a subnet with largely static PCs, longer leases may be more appropriate. 
  3. DHCP servers supporting DHCP authorization in Active Directory: When a client device issues a DHCP request, all listening servers respond with an offer and the client responds to the first answer received. This is because DHCP requests are broadcast-based. Therefore, having DHCP servers that support DHCP authorization in Active Directory prevents the use of rogue DHCP servers. 
  4. Configuring DHCP for Redundancy: For redundancy, it’s advisable to configure two DHCP servers and split the scope using a 50:50 or 80:20 ratio. If both DHCP servers are located in the same site, the 50:50 ratio would work well. If redundancy is provided by a remote site, then the 80:20 ratio may prove more useful. Thus, in a situation where one DHCP server goes down, the other takes care of providing clients with IP addresses. However, it should be noted that DHCP servers don’t coordinate or check if the assigned IP addresses are unique. So, network administrators should be extra careful when dividing the IP address pool among servers, to avoid duplicate address assignments that lead to IP conflict issues. 
  5. Using Superscopes: When two DHCP servers service the same subnet, but manage a different scope of IP addresses, it should be noted that neither will have information regarding the others respective managed addresses. In such a case, when a DHCP client broadcasts its renewal request, the particular server that first responded to the client will be unavailable. Subsequently, the renewal will fail and the client will continue to delay while trying to renew its lease. Eventually, it will enter the rebinding state. Thereafter, the client broadcasts to the subnet trying to locate a valid IP configuration. Then the second DHCP server might respond, but with a DHCP negative acknowledgement message (DHCPNAK) in reply, as it does not recognize the client’s IP renewal request. 

    To avoid such problems when using more than one DHCP server on the same subnet, use a DHCP superscope configured similarly at all servers. Superscope is an administrative feature that enables grouping of multiple scopes as a single entity. The superscope should include all valid scopes for the subnet as member scopes.  This way, the DHCP server can activate and provide leases for more than one scope to clients on a single physical network.

     

  6. Deactivating Scopes – Once a scope is activated, it shouldn’t be deactivated until you‘re ready to retire the scope and associate IP addresses from use on the network. Deactivating a scope causes undesired DHCPNAK to be sent to the clients leased in the scope.

Network administrators should remember to not leave out planning for interoperability between DHCP, DNS, and WINS services. Specifically, this helps reduce room for error and downtime due to changes made within DHCP settings. Whether you’re using Microsoft DHCP or have a multi-vendor DHCP environment, you can deploy an IP address management software overlay to simplify, centralize, and automate your entire IP address management efforts.

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  • Nash

    > organizations tend to use static IPs for devices like servers…

    You mean DHCP Reservations, right? Static IP addresses hand configured on devices is so 1992…