Cisco DHCP

1. Using IP Helper Addresses for DHCP
Router#configure terminal
Router1(config)#interface Ethernet 0
Router1(config-if)#ip helper-address 172.25.1.1
Router1(config-if)#ip helper-address 172.25.10.7
Router1(config-if)#exit
Router1(config)#end
Router1#

The ip helper-address configuration command allows the router to forward local DHCP requests to one or more centralied DHCP servers.

2. Limiting the impact of IP Helper Address
Router1# configure terminal
Router1(config)#no ip forward-protocol udp tftp
Router1(config)#no ip forward-protocol udp nameserver
Router1(config)#no ip forward-protocol udp domain
Router1(config)#no ip forward-protocol udp time
Router1(config)#no ip forward-protocol udp netbios-ns
Router1(config)#no ip forward-protocol udp netbios-dgm
Router1(config)#no ip forward-protocol udp tacacs
Router1(config)#end
Router1#

After configuring IP helper addresses, you suffer from high link utilization / high CPU utilization on the DHCP server.

3. Using DHCP to Dynamically Configure Router IP address
Router1#configure terminal
Router1(config)#interface FastEthernet0/1
Router1(config-ig)#ip address dhcp
Router1(config-if)#end
Router1#

The ip address dhcp configuration command allows ther router to obtaion the address information for an interface dynamically.

Router1#show ip interface
FastEthernet0/1 is up, line protocol is up
    Internet address is 172.25.1.57/24
    Broadcast address is 255.255.255.255
    Address determined by DHCP
    MTU is 1500 bytes
    <removed for brevity>

4. Dynamically allocating Client IP address via DHCP

5. Defining DHCP Configuration options
Router1#configure terminal
Router1(config)#ip dhcp pool ORAserver
Router1(dhcp-config)#host 172.25.1.34 255.255.255.0
Router1(dhcp-config)#client-name bigserver
Router1(dhcp-config)#default-router 172.25.1.1 172.25.1.3
Router1(dhcp-config)#domain-name oreilly.com
Router1(dhcp-config)#dns-server 172.25.1.1 10.1.2.3
Router1(dhcp-config)#netbios-name-server 172.25.1.1
Router1(dhcp-config)#netbios-node-type h-node
Router1(dhcp-config)#option 66 ip 10.1.1.1
Router1(dhcp-config)#option 33 ip 192.0.2.1 172.25.1.3
Router1(dhcp-config)#option 31 hex 01
Router1(dhcp-config)#lease 2
Router1(dhcp-config)#exit
Router1(config)#end
Router1#

RFC 2132 defines a large number of standard configurable options.

 6. Allocating static IP address with DHCP
You want to configure 
Router1#configure terminal
Router1(config)#ip dhcp pool IAN
Router1(dhcp-config)#host 172.25.1.13 255.255.255.0
Router1(dhcp-config)#client-identifier 0100.0103.85e9.87
Router1(dhcp-config)#client name win2k
Router1(dhcp-config)default-router 172.25.1.1
Router1(dhcp-config)#exit
Router1(config)#end
Router1#

7. Configuring a DHCP database client
You want to backup your database DHCP address assignment to another device so that you won't lose it if the router reloads.

Router1#configure terminal
Router1(config)#


8. Configuring multiple dhcp server per 1 subnet
You want to
Router1# configure terminal
Router1(config)#ip dhcp pool 172.22.1.0/24
Router1(dhcp-config)# network 172.22.1.0 255.255.255.0
Router1(dhcp-config)# domain-name Oreilly.com