Troubleshooting dhcp problems
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Troubleshooting dhcp problems



Troubleshooting dhcp problems

Troubleshooting dhcp problems



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Troubleshooting dhcp problems Document Transcript

  • 1. Troubleshooting DHCP ProblemsOne of the most important services that run on an organization’s network is theDHCP server. Without a correctly configured DHCP server to supply the address andserver information to be used on each of the client devices, users are left without ausable network computer. If this happens the administrator only has a few options:get the server back up and running correctly or statically configure each of the clientdevices; the latter not being preferred. This article takes a look at a number of thecommon DHCP problems and their symptoms from the perspective of the networkadministrator and the DHCP server (being run on a Cisco device).Common DHCP Configuration ProblemsThere are a couple common configuration problems that can make the life of anetwork administrator quite challenging, these include: Server not giving out addresses Clients receiving addresses already statically assigned to servers or reserved devices Clients unable to reach external networks (off subnet) Clients unable to use the Internet with domain names Clients not receiving domain name suffixServer Not Giving out AddressesThere are a couple of reasons that a server (In this case the Cisco device) will not giveout addresses. The most obvious of these is that the server is not configured. On aCisco DHCP device, this can be verified by viewing the current DHCP poolconfiguration using the show ipdhcp pool command; this is shown in Figure 1.Figure 1 - No DHCP Pool Configured
  • 2. second reason is that the DHCP pool configured does not match any of theinterfaces that are currently configured on the Cisco device. An example of this isshown in Figure 2. In this example, R1 only has a single interface (f0/0) that isconfigured with an IP address of and a single DHCP pool that isconfigured to give out the addresses from the network.Figure 2 - No Matching InterfaceClients Receiving Addresses Already Statically Assigned to Servers or ReservedDevicesA common mistake that happens when configuring a Cisco device as a DHCP server isthe failure to reserve addresses that are already in use (including the device itself).By default, when configuring a DHCP pool the whole range of addresses are eligiblefor lease. Figure 3 shows an example of this misconfiguration. In this example, theaddress has already been statically configured on a local server.
  • 3. 3 - DHCP Excluded Address MisconfigurationKeep in mind, however, that by default the Cisco DHCP server will attempt to pingthe next IP address in the pool to ensure the IP address is not currently being used.The example shown in Figure 3 is only possible if the local server was not on whenthe address was requested or did not respond to the ping request from the DHCPServer.Clients Unable to Reach External Networks (off subnet)Another common mistake happens when a DHCP server has been configured butclients are unable to reach networks outside the local subnet. When setting up aconfiguration for client devices that will need to reach clients external to the localsubnets (almost all modern clients), a default gateway must be set up inside theDHCP configuration. On a Cisco device, the configuration of the default gatewayinside a DHCP pool is done with the default-routercommand. The example in Figure4 shows a DHCP pool configuration with a default gateway.Figure 4 - IP DHCP Server Default Gateway ConfigurationClients Unable to Use the Internet with Domain NamesTypically, incorrect configuration of either the default gateway or the inability to usethe Internet using Domain Names are grouped into the same problem category. Forusers to be able to use the Internet using domain names, a Domain Name Server(DNS) server (or servers) must be configured on their local machines. It is possible toconfigure these separately from the IP addressing, but in most typical configurationsthe DNS server (or servers) is configured as part of the DHCP pool. On a Cisco device,the configuration of a DNS server (or servers) is done with the dns-server command.The example in Figure 5 shows a DHCP pool configuration with DNS serversconfigured.
  • 4. 5 – IP DHCP Server DNS ConfigurationClients Not Receiving Domain Name SuffixAn often overlooked function of a DHCP server is that is can also give out a domainname suffix. If the organizational network relies on both DHCP and DNS, users canbecome accustom to using single name domain names for local servers; for examplemarketing (for Without a correctly configured domain namesuffix, the users would need to type out the complete domain name (; depending on the temperament of the users inside the organizationthis can become a problem. On a Cisco device, the configuration of a domain namesuffix is done with the domain-name command. The example in Figure 6 shows aconfiguration for a domain name suffix.Figure 6 - IP DHCP Server Domain Name Suffix Configuration
  • 5. are certainly a number of advantages of using DHCP inside an organization; forthe most part once DHCP is up and running within an organization it is rock solid andperforms very well. However, if the configuration is not quite correct many differentDHCP problems can occur as a result. Proper configuration is a must to avoid theseDHCP problems, but the real focus of this article is for those situations when theconfiguration has already been completed and issues are being troubleshooted.Hopefully, the information contained with this article will be able to be practicallyused to help fix these common misconfigurations.More DHCP Info:DHCP & DHCP OperationHow to Configure DHCP on a Cisco Router or Cisco Switch?Tutorial: How to Add a DHCP Range to a Cisco 3750 Switch?