Cisco IOS Cookbook, 2nd Edition

Dynamically Allocating Client IP Addresses via DHCP

Problem

You want to configure your router to be a DHCP server and allocate dynamic IP addresses to client workstations.

Solution

The following set of configuration commands allows the router to dynamically allocate IP addresses to client workstations:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#service dhcp
Router1(config)#ip dhcp pool 172.25.1.0/24
Router1(dhcp-config)#network 172.25.1.0 255.255.255.0
Router1(dhcp-config)#default-router 172.25.1.1
Router1(dhcp-config)#exit
Router1(config)#ip dhcp excluded-address 172.25.1.1 172.25.1.50
Router1(config)#ip dhcp excluded-address 172.25.1.200 172.25.1.255
Router1(config)#end
Router1#

Discussion

Cisco incorporated DHCP server functionality starting in IOS Version 12.0(1)T. This allows routers to dynamically allocate IP addresses to client workstations without needing a centralized DHCP server.

Providing DHCP services from a router has some interesting advantages over using a central server. First, distributing the DHCP functionality into the access routers of a large network reduces the risk of a server configuration problem affecting the entire corporate network. Second, maintaining DHCP services within each remote branch router reduces the utilization over expensive WAN links. Third, when a WAN link fails, workstations are still able to function on the local segment. And fourth, the router based DHCP services can easily be administered ...