Network topology considerations

Understanding your network topology

One of the most important aspects of planning a DHCP implementation is understanding you network layout or topology. When you understand your network topology, you will be able to quickly identify the IP address ranges for DHCP, the configuration information that each client needs, the devices that need to be configured to forward DHCP messages, and whether DHCP can work with your DNS or PPP servers. Depending on the complexity of your network, you might even want to sketch your network topology on a piece of scrap paper. You must include all of the LANs, the devices that connect the LANs, and the IP addresses for devices and clients (for example, a printer) that need a defined IP address. You might want to look at some of the DHCP scenarios to help you sketch out your network topology.

Determining the number of DHCP servers

Even with a complex network, you can still manage all of your network clients using only one DHCP server. Depending on your network topology, you might need to set up a few DHCP/BOOTP relay agents or enable your routers to forward DHCP packets to make it work.

Using only one DHCP server for your entire network will centralize host configuration management for all of your clients. However, there are cases where you might want to consider using multiple DHCP servers in your network.

To avoid a single point of failure, you can configure two or more DHCP servers to serve the same subnet. If one server fails, the others can continue to serve the subnet. Each of the DHCP servers must be accessible either by direct attachment to the subnet or by using a DHCP/BOOTP relay agent.

Because two DHCP servers cannot serve the same addresses, address pools defined for a subnet must be unique across DHCP servers. Therefore, when using two or more DHCP servers to serve a particular subnet, the complete list of addresses for that subnet must be divided among the servers. For example, you can configure one server with an address pool consisting of 70% of the available addresses for the subnet and the other server with an address pool consisting of the remaining 30% of the available addresses.

Using multiple DHCP servers decreases the probability of having a DHCP-related network access failure, but it does not guarantee against it. If a DHCP server for a particular subnet fails, the other DHCP server might not be able to service all the requests from new clients, which might, for example, exhaust the server's limited pool of available addresses.

If you are considering multiple DHCP servers, remember that multiple DHCP servers cannot share any of the same addresses. If you use more than one DHCP server in your network, each server must be configured with their own unique IP address ranges.

Identifying the IP addresses that your DHCP server should manage

Using your network topology, you can document which network address ranges you want the DHCP server to manage. You must identify which devices have a manually configured IP address (for example, the router's IP address) that you want to exclude from the DHCP's address pool.

In addition, you will want to consider whether these addresses should be assigned dynamically by the DHCP server, or you want to assign specific IP addresses to certain clients. You might want to reserve a specific address and configuration parameters for a specific client on a particular subnet, such as a file server. Or, you might want to map all of your clients to a specific IP address. Refer to DHCP client support for more information about assigning IP addresses dynamically versus statically.

Determining the lease time for the IP addresses

The default lease time for the DHCP server is 24 hours. The duration for which you set the lease time on your DHCP server depends on several factors. You will need to consider your goals, your site's usage patterns, and service arrangements for your DHCP server. For more information to help you determine the lease time for your DHCP clients, refer to Leases.

Supporting BOOTP clients

If you are currently using a BOOTP server, consider that the DHCP server can replace the BOOTP server on your network with little or no impact to your BOOTP clients. There are three options for you if you have BOOTP clients currently on your network.

The easiest option is to configure your DHCP server to support BOOTP clients. When you use DHCP to support your BOOTP clients, each BOOTP client is essentially being mapped to a single IP address, and that address is therefore not re-usable by another client. The advantage, however, of using DHCP in this case is that there is no need to configure a one-to-one mapping of BOOTP clients to IP addresses. The DHCP server will still dynamically assign an IP addresses to the BOOTP clients from the address pool. After the IP address is assigned to the BOOTP client, it is permanently reserved for use by that client until you explicitly delete the address reservation. This is a good option if you have a large number of BOOTP clients in your network.

Another option is to migrate your BOOTP server configuration to the DHCP server. A DHCP client will be created for each BOOTP client listed in the BOOTP server configuration. In this option, it is recommended that you reconfigure your clients to be DHCP clients. However, when you migrate your BOOTP configuration to DHCP, the DHCP address assignments will work for either a BOOTP or DHCP client. This might be a good option to transition your BOOTP clients to DHCP. Your BOOTP clients will still be supported during the process of reconfiguring them to DHCP.

Eventually, you might want to do the third option: change each BOOTP client to DHCP and configure DHCP to dynamically assign them addresses. Essentially, this option removes BOOTP entirely from the network.

Identifying the configuration information for the network clients

Using your network topology layout, you can clearly see the devices (for example, routers) that must be identified in the DHCP configuration. In addition, you must identify other servers in your network, such as the Domain Name System (DNS) server, that your clients might need to know about. You can either specify this information for the entire network, a specific subnet, or a specific client regardless of the subnet.

If you have devices that apply to many clients, you will want to specify them at the highest level possible (for example, at the Global level for the entire network, or at the subnet level for a specific subnet). This will minimize the changes you will need to make to the DHCP configuration when the device changes. If you have specified the same router, for example, for every client in your network, you must change the configuration for every client when the router has changed. However, if you have specified the router at the global level (all of the clients will inherit this configuration information), you only need to change the information once and the information is changed for all clients.

Some of your clients might have unique TCP/IP configuration requirements that require the information to be configured at the client level. DHCP can recognize those clients and provide the unique configuration data to them. This is not only true for the configuration options, but also for the lease time and IP address. For example, a client might need a longer lease time than all the other clients. Or, maybe only one client, such as a file server, needs a dedicated IP address. Identifying those clients and the unique information they require beforehand will help you when you start configuring the DHCP server.

For a quick reference to all of the configuration options, refer to DHCP options lookup.

Using dynamic DNS with your DHCP server

If you are currently using a DNS server to manage all of your client's host names and IP addresses, you will definitely want to reconfigure your DNS server to accept dynamic updates from DHCP. If you use Dynamic DNS, the clients will not notice any interruption or changes in the DNS service when you switch over to DHCP. For more information about using DHCP with your DNS server, refer to Dynamic updates.

If you are not currently using a DNS server, you might want to consider adding a DNS server when you add the DHCP server. You can read the DNS topic in the information center to find out more about DNS benefits and requirements.

Using DHCP for your remote clients

If you have any remote clients that connect to your network using PPP, you can set up DHCP to dynamically assign an IP address to those remote clients when they connect to the network. To see some scenarios of networks where this might be useful, see Scenario: PPP and DHCP on a single IBM i or Scenario: DHCP and PPP profile on different IBM i models. These scenarios also explain how to set up the network to use PPP and DHCP together for your remote clients.