Configure DHCP on a Cisco router or switch

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If you want to configure Dynamic Host Configuration Protocol
(DHCP) on PC clients, you currently have multiple options to choose for your DHCP server. One of the more
common ways to accomplish this is by using a Windows or Linux server. However,
most home networks get DHCP from their DSL or cable router.

Many administrators forget—or don’t even realize—that DHCP
is also available on Cisco IOS routers and switches. Keep in mind that DHCP is only
available on newer IOS-based switches. For example, Catalyst 3550 and 3750
offer DHCP.

Deciding if this is right for your organization

Using a router as a DHCP server has its pros and cons. For
example, let’s say you have a multisite network, and you’ve decided to use your
routers as DHCP servers for each network.

One benefit of running DHCP on a routers is that it doesn’t
require any extra hardware (like a Windows server does). In addition, running
DHCP on a router can save your organization the cost of a dedicated DHCP
server. It also provides DHCP locally at each site—regardless of whether the
WAN is operational.

But there are some drawbacks. For example, some
administrators might not be too comfortable monitoring and troubleshooting DHCP
from a router; they may prefer to use Windows DHCP Manager rather than Cisco
IOS commands to check the status of client leases, manually terminate a DHCP
lease, or assign a static reservation. In addition, some administrators might
prefer implementing a centralized DHCP server in order to have one place to
monitor and troubleshoot DHCP.

Of course, every IT shop is different, with varying skill
sets, comfort levels, and budgets. Depending on your organization’s needs, Cisco
IOS DHCP could be the perfect fit for your network.

Getting started

Let’s look at how to configure basic DHCP on an IOS-based
router. For this example, we’ll start off with the default configuration on a
Cisco 2611 router running IOS 12.2. (The configuration should be the same—or
very similar—on all IOS-based routers).

To begin, connect the router’s Ethernet port to a switch,
and connect the switch to a laptop, which will serve as the DHCP client.

To configure Cisco IOS DHCP, follow these steps, which
include sample commands:

1. Configure
   an IP address on the router’s Ethernet port, and bring up the interface. (On
   an existing router, you would have already done this.)

Router(config)# interface ethernet0/0

Router(config-if)#ip address 1.1.1.1 255.0.0.0

Router(config-if)# no shutdown

2. Create
   a DHCP IP address pool for the IP addresses you want to use.

Router(config)# ip dhcp pool mypool

3. Specify
   the network and subnet for the addresses you want to use from the pool.

Router(dhcp-config)# network 1.1.1.0 /8   

4. Specify
   the DNS domain name for the clients.

Router(dhcp-config)#domain-name mydomain.com

5. Specify
   the primary and secondary DNS servers.

Router(dhcp-config)#dns-server 1.1.1.10 1.1.1.11

6. Specify
   the default router (i.e., default gateway).

Router(dhcp-config)#default-router 1.1.1.1

7. Specify
   the lease duration for the addresses you’re using from the pool.

Router(dhcp-config)#lease 7

8. Exit
   Pool Configuration Mode.

Router(dhcp-config)#exit

This takes you back to the global configuration prompt. Next,
exclude any addresses in the pool range that you don’t want to hand out.

For example, let’s say that you’ve decided that all IP addresses
up to .100 will be for static IP devices such as servers and printers. All IP
addresses above .100 will be available in the pool for DHCP clients.

Here’s an example of how to exclude IP addresses .100 and
below:

Router(config)#ip dhcp excluded-address 1.1.1.0 1.1.1.100

Next, enter the ipconfig
/renew command on the laptop to receive an IP address. After you have the
IP address, enter the ipconfig /all
command. Listing A shows sample
output from this command.

Using DHCP commands

After configuring DHCP on the router, you can use DHCP show commands to see what’s going on. For
example, you can use one of the most common DHCP commands to view which DHCP IP
addresses currently have leases: show ip
dhcp bindings.

Listing B shows
sample output from this command. From this output, you can see that my PC
received the IP address of 1.1.1.101 (the first address after 100) and its MAC
address.

Keep in mind that DHCP configuration can be very complex on
Cisco routers. You can configure backup servers, settings to prevent conflicts,
secure DHCP, and many other options. For more information, check out Cisco’s
Configuring DHCP documentation.

David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.