DHCP Topic

DHCP Topic

DHCP Operation
1. Client locate DHCP Server, sends DHCPDISCOVER (broadcast)
2. Server replies with DHCPOFFER (unicast)
3. Client sends DHCPREQUEST (broadcast)
4. Server sends DHCPACK to client (unicast)

DHCPDISCOVER –> DHCPOFFER –> DHCPREQUEST -> DHCPACK
broadcast -> unicast -> broadcast -> unicast

Note: client always broadcast, Server always unicast

DHCP Mechanisms
a. Manual – assigns the IP address to MAC Address
b. Automatic – IP address is permanently assigned to a host
c. Dynamic – IP address is assigned to host for a limited time


Configuring a Router to be a DHCP Client
Router(config)#interface Fast 0/1
Router(config-if)#ip address dhcp
Note: configured on an interface, think nic on computer.

Limiting the request info
Router(config-if)#no ip dhcp request dns-nameserver
Note:by default all services are available

Useful commands
Router#show host
Router#release dhcp Fa0/1
Router#renew dhcp Fa0/1
Router#show dhcp lease


Configuring IOS for DHCP Server
Router(config)#service dhcp
Note: on by default
Router(config)#no service dhcp

Define pool of address to be given to host
Sales Department: 172.16.1.0/24 with 172.16.1.1 to 172.16.10 excluded
Router(config)#ip dhcp pool sales-dept
Router(dhcp-config)#network 172.16.1.0 255.255.255.0

Router(dhcp-config)#ip dhcp excluded-address 172.16.1.1 172.16.1.10

Changing key info.
Router(config)#ip dhcp pool sales-dept
Router(dhcp-config)#dns-server 172.16.1.2
Router(dhcp-config)#netbios-name-server 172.16.1.2
Router(dhcp-config)#default-router 172.16.1.1
Router(dhcp-config)#domain-name jepoy.net


IP Helper Address
Purpose: accepts broadcast request and forward it as unicast to a specific dhcp server

Example: Forward all DHCP request to 172.16.1.3 DHCP server
Router(config)#interface FastEthernet 0/0
Router(config-if)#ip helper-address 172.16.1.3

Limiting the impact of ip helper address
Example: Remove tftp and time
Router(config)#no ip forward-protocol udp tftp
Router(config)#no ip forward-protocol udp time

Example:Add UDP 517
Router(config)#ip forward-protocol udp 517


8-Default Forward Service
Port Service Name
37 Time
49 TACACS
53 DNS
67 DHCP Client
68 DHCP Server
69 TFTP
137 NETBIOS name service
138 NETBIOS datagram service