Cisco IOS routers and layer 3 switches can be configured as DHCP servers. It’s quite easy to do this and in this short lesson, I want to explain to you how to do this and how to verify your configuration. If you are a little fuzzy about how DHCP works, take a look at my introduction to DHCP first.
Let’s use the following topology:
Above, we have a router that I will call ‘DHCP’. The router and computer are connected to each other using a simple switch and in the same VLAN. We will use the 192.168.12.0 /24 subnet for this demonstration. Let’s prepare the interface first:
DHCP(config)#interface fastEthernet 0/0 DHCP(config-if)#no shutdown DHCP(config-if)#ip address 192.168.12.1 255.255.255.0
Now let’s configure the DHCP server:
DHCP(config)#ip dhcp pool MYPOOL DHCP(dhcp-config)#network 192.168.12.0 255.255.255.0
ip dhcp pool command to create a DHCP pool and give it a name. This DHCP pool will use network 192.168.12.0 /24. Basically, this is all you have to do to get the DHCP server going, there is no need to start a service or something. We can verify that we have DHCP clients using the following command:
DHCP#show ip dhcp binding Bindings from all pools not associated with VRF: IP address Client-ID/ Lease expiration Type Hardware address/ User name 192.168.12.2 0063.6973.636f.2d63. Mar 02 2002 12:24 AM Automatic 6330.372e.3132.3265. 2e30.3030.302d.4661. 302f.30
Above, you can see that we have a DHCP client that received IP address 192.168.12.2. In production networks, we will also use DHCP to hand out some other useful things like a default gateway, DNS server, and more. Let’s see how we can do this:
DHCP(config)#ip dhcp pool MYPOOL DHCP(dhcp-config)#default-router 192.168.12.1 DHCP(dhcp-config)#dns-server 220.127.116.11
Above, I configured the IP address 192.168.12.1 as the default gateway for the DHCP clients with the
default-router command. The
dns-server commands let us specify a DNS server.
Something else you might want to do is exclude some IP addresses. With the configuration so far, our DHCP server will hand out IP addresses .2,3,4,5,6, etc. Here’s how to do it: