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212 lines
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<body id="rzakgplanningdemands"><a name="rzakgplanningdemands"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Network topology considerations</h1>
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<div><p>You can plan for most of the DHCP setup by looking at your network
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topology, the devices on the network (for example, routers), and how you want
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to support your clients in DHCP.</p>
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<div class="section"><h4 class="sectiontitle">Understand your network topology</h4><p>One of the most
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important aspects of planning a DHCP implementation is understanding you network
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layout or topology. When you understand your network topology, you will be
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able to quickly identify the IP address ranges for DHCP, the configuration
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information that each client needs, the devices that need to be configured
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to forward DHCP messages, and whether DHCP can work with your DNS or PPP servers.
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Depending on the complexity of your network, you might even want to sketch
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your network topology on a piece of scrap paper. You should include all of
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the LANs, the devices that connect the LANs, and the IP addresses for devices
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and clients (for example, a printer) that need a defined IP address. You might
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want to look at some of the DHCP examples to help you sketch out your network
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topology.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Determine the number of DHCP servers</h4><p>Even with a
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complex network, you can still manage all of your network clients using only
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one DHCP server. Depending on your network topology, you might need to set
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up a few DHCP/BOOTP relay agents or enable your routers to forward DHCP packets
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to make it work. </p>
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<p>Using only one DHCP server for your entire network
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will centralize host configuration management for all of your clients. However,
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there are cases where you might want to consider using multiple DHCP servers
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in your network.</p>
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<p>To avoid a single point of failure, you can configure
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two or more DHCP servers to serve the same subnet. If one server fails, the
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other can continue to serve the subnet. Each of the DHCP servers must be accessible
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either by direct attachment to the subnet or by using a DHCP/BOOTP relay agent.</p>
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<p>Because
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two DHCP servers cannot serve the same addresses, address pools defined for
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a subnet must be unique across DHCP servers. Therefore, when using two or
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more DHCP servers to serve a particular subnet, the complete list of addresses
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for that subnet must be divided among the servers. For example, you can configure
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one server with an address pool consisting of 70% of the available addresses
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for the subnet and the other server with an address pool consisting of the
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remaining 30% of the available addresses.</p>
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<p>Using multiple DHCP servers
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decreases the probability of having a DHCP-related network access failure,
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but it does not guarantee against it. If a DHCP server for a particular subnet
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fails, the other DHCP server might not be able to service all the requests
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from new clients, which might, for example, exhaust the server's limited pool
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of available addresses.</p>
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<p>If you are considering multiple DHCP servers,
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remember that multiple DHCP servers cannot share any of the same addresses.
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If you use more than one DHCP server in your network, each server must be
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configured with their own unique IP address ranges.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Identify the IP addresses that your DHCP server should manage</h4><p>Using
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your network topology, you should start documenting which network address
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ranges you want the DHCP server to manage. You should identify which devices
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have a manually configured IP addresses (for example, the router's IP address)
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that you want to exclude from the DHCP's address pool.</p>
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<p>In addition,
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you will want to consider whether these addresses should be assigned dynamically
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by the DHCP server or if you want to assign a specific IP address to certain
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clients. You might want to reserve a specific address and configuration parameters
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for a specific client on a particular subnet, such as a file server. Or, you
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might want to map all of your clients to a specific IP address. Refer to DHCP
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client support for more information about assigning IP addresses dynamically
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versus statically.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Determine the lease time for the IP addresses</h4><p>The
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default lease time for the DHCP server is 24 hours. The duration for which
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you set the lease time on your DHCP server depends on several factors. You
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will need to consider your goals, your site's usage patterns, and service
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arrangements for your DHCP server. For more information to help you determine
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the lease time for your DHCP clients, refer to Leases.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Support BOOTP clients</h4><p>If you are currently using
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a BOOTP server, consider that the DHCP server can replace the BOOTP server
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on your network with little or no impact to your BOOTP clients. There are
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three options for you if you have BOOTP clients currently on your network.</p>
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<p>The
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easiest option is to configure your DHCP server to support BOOTP clients.
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When you use DHCP to support your BOOTP clients, each BOOTP client is essentially
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being mapped to a single IP address, and that address is therefore not re-usable
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by another client. The advantage, however, of using DHCP in this case is that
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there is no need to configure a one-to-one mapping of BOOTP clients to IP
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addresses. The DHCP server will still dynamically assign an IP addresses to
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the BOOTP client from the address pool. After the IP address is assigned to
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the BOOTP client, it is permanently reserved for use by that client until
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you explicitly delete the address reservation. This is a good option if you
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have a large number of BOOTP clients in your network.</p>
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<p>Another option
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is to migrate your iSeries™ BOOTP server configuration to the DHCP server.
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A DHCP client will be created for each BOOTP client listed in the BOOTP server
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configuration. In this option, it is recommended that you reconfigure your
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clients to be DHCP clients. However, when you migrate your BOOTP configuration
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to DHCP, the DHCP address assignments will work for either a BOOTP or DHCP
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client. This might be a good option to transition your BOOTP clients to DHCP.
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Your BOOTP clients will still be supported during the process of reconfiguring
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them to DHCP.</p>
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<p>Eventually, you might want to do the third option: change
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each BOOTP client to DHCP and configure DHCP to dynamically assign them addresses.
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Essentially, this option removes BOOTP entirely from the network.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Identify the configuration information for the network clients</h4><p>Using
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your network topology layout, you can clearly see the devices (for example,
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routers) that must be identified in the DHCP configuration. In addition, you
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should identify other servers in your network, such as the Domain Name System
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(DNS) server, that your clients might need to know about. You can either specify
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this information for the entire network, a specific subnet, or a specific
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client regardless of the subnet.</p>
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<p>If you have devices that apply to many
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clients, you will want to specify them at the highest level possible (for
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example, at the Global level for the entire network, or at the subnet level
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for a specific subnet). This will minimize the changes you will need to make
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to the DHCP configuration when the device changes. If you have specified the
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same router, for example, for every client in your network, you need to change
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the configuration for every client when the router has changed. However, if
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you have specified the router at the global level (all of the clients will
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inherit this configuration information), you only need to change the information
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once and the information is changed for all clients.</p>
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<p>Some of your clients
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might have unique TCP/IP configuration requirements that requires information
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to be configured at the client level. DHCP can recognize those clients and
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provide the unique configuration data to them. This is not only true for the
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configuration options, but also for the lease time and IP address. For example,
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a client might need a longer lease time that all of the other clients. Or,
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maybe only one client, such as a file server, needs a dedicated IP address.
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Identifying those clients up front and what unique information they require
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will help you when you start configuring the DHCP server.</p>
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<p>For
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a quick reference to all of the configuration options, refer to <a href="rzakgconceptoptions.htm#rzakgconceptoptions">DHCP options lookup</a>.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Use dynamic DNS with your DHCP server</h4><p>If you are
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currently using a DNS server to manage all of your client's host names and
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IP addresses, you will definitely want to reconfigure your DNS server to accept
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dynamic updates from DHCP. If you use Dynamic DNS, the clients will not notice
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any interruption or changes in the DNS service when you switch over to DHCP.
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For more information about using DHCP with your DNS server, refer to Dynamic
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updates.</p>
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<p>If you are not currently using a DNS server, you might want
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to consider adding a DNS server when you add the DHCP server. You can read
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the DNS topic to find out more about DNS benefits and requirements in the
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information center.</p>
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</div>
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<div class="section"><h4 class="sectiontitle">Use DHCP for your remote clients</h4><p>If you have any
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remote clients that connect to your network using PPP, you can set up DHCP
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to dynamically assign an IP address to them when they connect to the network.
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To see some examples of networks where this might be useful, see Example:
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PPP and DHCP on a single iSeries Server or Example: DHCP and PPP profile on
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different iSeries servers.
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These examples also explain how to set up the network to use PPP and DHCP
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together for your remote clients.</p>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rzakgplanning.htm" title="There are several steps you must take to plan how DHCP should be set up for your network.">Plan for DHCP</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="rzakgconceptrelayrouter.htm" title="You might need to use a DHCP relay agent in your network; sometimes a router is sufficient. You can use both a DHCP relay agent and a router to efficiently and securely transfer data throughout the network.">Relay agents and routers</a></div>
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<div><a href="rzakgexampleparent.htm" title="By reviewing diagrams and examples of how different networks are set up, you can determine which is the best choice for your installation.">DHCP examples</a></div>
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<div><a href="rzakgconceptsupport.htm" title="Using DHCP, you can manage each client in your network individually, rather than managing all of the clients as a large group (subnet).">DHCP client support</a></div>
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<div><a href="rzakgconceptleases.htm" title="You can find what DHCP leases are and some questions are provided for you to consider when determining the lease time for your DHCP clients.">Leases</a></div>
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<div><a href="rzakgconceptbootp.htm" title="This topic describes what BOOTP is, and gives some history about BOOTP and DHCP.">BOOTP</a></div>
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<div><a href="rzakgconceptddns.htm" title="You can use your DHCP server in conjunction with your DNS server to dynamically update the client information in the DNS when DHCP assigns the client an IP address.">Dynamic updates</a></div>
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<div><a href="../rzakk/rzakkkickoff.htm">DNS</a></div>
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<div><a href="rzakgproblemnoip.htm" title="Problems might occur if the clients cannot receive an IP address or the configuration information. An IP address is leased to a client through a four-step process between the client and the DHCP server.">Problem: Clients are not receiving an IP address or their configuration information</a></div>
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</div>
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<div class="relref"><strong>Related reference</strong><br />
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<div><a href="rzakgconceptoptions.htm" title="DHCP has many configuration options that can be sent to the client when they request information from the DHCP server. You can use a lookup tool that describes all of the DHCP options.">DHCP options lookup</a></div>
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<div><a href="rzakgscenario5.htm" title="You can learn how to set up the iSeries server as a DHCP server for a LAN and a remote dial-in client.">Example: PPP and DHCP on a single iSeries server</a></div>
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<div><a href="rzakgscenario6.htm" title="You can learn how to set up two iSeries servers as the network DHCP server and a DHCP/BOOTP relay agent for two LANs and remote dial-in clients.">Example: DHCP and PPP profile on different iSeries servers</a></div>
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</div>
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</div>
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