Troubleshooting dhcp problems

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Troubleshooting DHCP Problems
One of the most important services that run on an organization’s network is the
DHCP server. Without a correctly configured DHCP server to supply the address and
server information to be used on each of the client devices, users are left without a
usable 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 client
devices; the latter not being preferred. This article takes a look at a number of the
common DHCP problems and their symptoms from the perspective of the network
administrator and the DHCP server (being run on a Cisco device).
Common DHCP Configuration Problems
There are a couple common configuration problems that can make the life of a
network 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 suffix
Server Not Giving out Addresses
There are a couple of reasons that a server (In this case the Cisco device) will not give
out addresses. The most obvious of these is that the server is not configured. On a
Cisco DHCP device, this can be verified by viewing the current DHCP pool
configuration using the show ipdhcp pool command; this is shown in Figure 1.
Figure 1 - No DHCP Pool Configured

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The second reason is that the DHCP pool configured does not match any of the
interfaces that are currently configured on the Cisco device. An example of this is
shown in Figure 2. In this example, R1 only has a single interface (f0/0) that is
configured with an IP address of 10.10.10.1/24 and a single DHCP pool that is
configured to give out the addresses from the 20.20.20.0/24 network.
Figure 2 - No Matching Interface
Clients Receiving Addresses Already Statically Assigned to Servers or Reserved
Devices
A common mistake that happens when configuring a Cisco device as a DHCP server is
the 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 eligible
for lease. Figure 3 shows an example of this misconfiguration. In this example, the
address 10.10.10.2/24 has already been statically configured on a local server.

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Figure 3 - DHCP Excluded Address Misconfiguration
Keep in mind, however, that by default the Cisco DHCP server will attempt to ping
the 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 when
the address was requested or did not respond to the ping request from the DHCP
Server.
Clients Unable to Reach External Networks (off subnet)
Another common mistake happens when a DHCP server has been configured but
clients are unable to reach networks outside the local subnet. When setting up a
configuration for client devices that will need to reach clients external to the local
subnets (almost all modern clients), a default gateway must be set up inside the
DHCP configuration. On a Cisco device, the configuration of the default gateway
inside a DHCP pool is done with the default-routercommand. The example in Figure
4 shows a DHCP pool configuration with a default gateway.
Figure 4 - IP DHCP Server Default Gateway Configuration
Clients Unable to Use the Internet with Domain Names
Typically, incorrect configuration of either the default gateway or the inability to use
the Internet using Domain Names are grouped into the same problem category. For
users 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 to
configure these separately from the IP addressing, but in most typical configurations
the 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 servers
configured.

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Figure 5 – IP DHCP Server DNS Configuration
Clients Not Receiving Domain Name Suffix
An often overlooked function of a DHCP server is that is can also give out a domain
name suffix. If the organizational network relies on both DHCP and DNS, users can
become accustom to using single name domain names for local servers; for example
marketing (for marketing.sample.com). Without a correctly configured domain name
suffix, the users would need to type out the complete domain name (including
sample.com); depending on the temperament of the users inside the organization
this can become a problem. On a Cisco device, the configuration of a domain name
suffix is done with the domain-name command. The example in Figure 6 shows a
configuration for a domain name suffix.
Figure 6 - IP DHCP Server Domain Name Suffix Configuration

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Summary
There are certainly a number of advantages of using DHCP inside an organization; for
the most part once DHCP is up and running within an organization it is rock solid and
performs very well. However, if the configuration is not quite correct many different
DHCP problems can occur as a result. Proper configuration is a must to avoid these
DHCP problems, but the real focus of this article is for those situations when the
configuration has already been completed and issues are being troubleshooted.
Hopefully, the information contained with this article will be able to be practically
used to help fix these common misconfigurations.
More DHCP Info:
DHCP & DHCP Operation
How to Configure DHCP on a Cisco Router or Cisco Switch?
Tutorial: How to Add a DHCP Range to a Cisco 3750 Switch?