Below I’m going to illustrate how to configure a DHCP server on a Cisco router running IOS.
In this example, I have two Cisco 2621xm’s running c2600-ik9s-mz.123-22.bin. Each router’s FastEthernet 0/0 interface is plugged into a Cisco 2950 switch (ports 1 & 2) on the same VLAN.
R1, which has an IP address of 192.168.1.1 (/24), will be our DHCP server. It will be configured to hand out IP addresses in the range from 192.168.1.100 through 192.168.1.254. It will also hand out information identifying itself as the default gateway and DNS server. Our local domain name will be “ciscolab.local”.
Configure R1’s interface
R1# configure terminal R1(config)# interface fastethernet 0/0 R1(config-if)# ip address 192.168.1.1 255.255.255.0 R1(config-if)# no shutdown R1(config-if)# exit
Here we see the FastEthernet 0/0 interface coming up…
*Mar 1 00:15:46.306: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up *Mar 1 00:15:47.308: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
Exclude addresses from the DHCP pool
Let’s tell the router not to give out the ip addresses from 192.168.1.1 through .99.
R1(config)# ip dhcp excluded-address 192.168.1.1 192.168.1.99
Create the DHCP pool
R1(config)# ip dhcp pool DHCP_POOL R1(dhcp-config)# network 192.168.1.0 255.255.255.0 R1(dhcp-config)# default-router 192.168.1.1 R1(dhcp-config)# dns-server 192.168.1.1 R1(dhcp-config)# domain-name ciscolab.local R1(dhcp-config)# end R1(config)#
Configure the DHCP client on R2
At this point, our work on R1 is done. Now let’s go over to R2, configure the FastEthernet 0/0 interface to use DHCP and bring it up.
R2# configure terminal R2(config)# interface fastethernet 0/0 R2(config-if)# ip address dhcp R2(config-if)# no shutdown
We see FastEthernet 0/0 come up.
*Mar 1 00:23:39.746: %LINK-3-UPDOWN: Interface FastEthernet0/0, changed state to up *Mar 1 00:23:40.748: %LINKPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/0, changed state to up
Shortly afterwards, we see that R2 has obtained an IP address.
*Mar 1 00:23:48.985: %DHCP-6-ADDRESS_ASSIGN: Interface FastEthernet0/0 assigned DHCP address 192.168.1.100, mask 255.255.255.0, hostname R2
R2(config-if)# end R2# show ip interface brief Interface IP-Address OK? Method Status Protocol FastEthernet0/0 192.168.1.100 YES DHCP up up FastEthernet0/0 unassigned YES unset administratively down down
R2#ping 192.168.1.1 Type escape sequence to abort. Sending 5, 100-byte ICMP echoes to 192.168.1.1, timeout is 2 seconds: !!!!! Success rate is 100 percent (5/5), round-trip min/avg/max = 4/11/16 ms
You can see a video of the whole process in action here: