Cisco - DHCP Relay Agent (IP Helper)

About

DHCP Relay Agent is the bridge between the client and the DHCP server. Through the DHCP Relay Agent, clients’ broadcasts can be received across different network segments, allowing the DHCP server to successfully assign IP addresses to clients.

Infrastructure

Configure multiple vlan groups on a single router interface and assign IP addresses through the DHCP server across different vlan groups.

PC1 in vlan10

PC2 in vlan20

PC3 in vlan30

PC0 in vlan40

DHCP Server (192.168.40.254) in vlan40, clients in all vlan (10, 20, 30, 40) can obtain IP addresses through the DHCP Server in vlan40.

DHCP Server Configuration

Set IP address to 192.168.40.254

Create 4 DHCP address pools corresponding to Vlan 10, Vlan 20, Vlan 30, and Vlan 40.

Router Configuration (Cisco 2911)

Router>enable
Router#configure terminal

Enter Global Configuration Mode

Router(config)#interface gigabitEthernet 0/0
Router(config-if)#no shutdown
Router(config-if)#exit

Select physical interface 0/0 and enable it

Router(config)#interface gigabitEthernet 0/0.10
Router(config-subif)#encapsulation dot1Q 10
Router(config-subif)#ip address 192.168.10.1 255.255.255.0
Router(config-subif)#exit

Configure sub-interface on physical interface 0/0

Configure 802.1Q protocol and assign vlan10

Set interface IP address ip address 192.168.10.1 255.255.255.0

Router(config)#interface gigabitEthernet 0/0.20
Router(config-subif)#encapsulation dot1Q 20
Router(config-subif)#ip address 192.168.20.1 255.255.255.0
Router(config-subif)#end

Configure sub-interface on physical interface 0/0

Configure 802.1Q protocol and assign vlan20

Set interface IP address ip address 192.168.20.1 255.255.255.0

Router(config)#interface gigabitEthernet 0/0.30
Router(config-subif)#encapsulation dot1Q 20
Router(config-subif)#ip address 192.168.30.1 255.255.255.0
Router(config-subif)#end

Configure sub-interface on physical interface 0/0

Configure 802.1Q protocol and assign vlan30

Set interface IP address ip address 192.168.30.1 255.255.255.0

Router(config)#interface gigabitEthernet 0/0.40
Router(config-subif)#encapsulation dot1Q 40
Router(config-subif)#ip address 192.168.40.1 255.255.255.0
Router(config-subif)#end

Configure sub-interface on physical interface 0/0

Configure 802.1Q protocol and assign vlan40

Set interface IP address ip address 192.168.40.1 255.255.255.0

Configure IP Helper (DHCP Relay Agent)

Router(config)#interface gigabitEthernet 0/0.10
Router(config-subif)#ip helper-address 192.168.40.254
Router(config-subif)#exit

Add the IP Helper (DHCP Relay Agent) to sub-interface 0/0.10, pointing to the DHCP Server at 192.168.40.254

Router(config)#interface gigabitEthernet 0/0.20
Router(config-subif)#ip helper-address 192.168.40.254
Router(config-subif)#exit

Add the IP Helper (DHCP Relay Agent) to sub-interface 0/0.20, pointing to the DHCP Server at 192.168.40.254

Router(config)#interface gigabitEthernet 0/0.30
Router(config-subif)#ip helper-address 192.168.40.254
Router(config-subif)#exit

Add the IP Helper (DHCP Relay Agent) to sub-interface 0/0.30, pointing to the DHCP Server at 192.168.40.254

Router(config)#interface gigabitEthernet 0/0.40
Router(config-subif)#ip helper-address 192.168.40.254
Router(config-subif)#exit

Add the IP Helper (DHCP Relay Agent) to sub-interface 0/0.40, pointing to the DHCP Server at 192.168.40.254

Router(dhcp-config)#end
Router#write

Save

Switch Settings (Cisco 2960-24T)

Switch1

Switch>enable
Switch#configure terminal

Enter Global Configuration Mode mode

Switch(config)#vlan 10
Switch(config-vlan)#vlan 20
Switch(config-vlan)#vlan 30
Switch(config-vlan)#vlan 40
Switch(config-vlan)#exit

Create 4 VLANs

Switch(config)#interface range gigabitEthernet 0/1-2
Switch(config-if-range)#switchport mode trunk
Switch(config-if-range)#exit

Define ports g0/1 and g0/2 as Trunk mode (*all VLAN data is transmitted through this interface to the client device)

Switch(config)#interface range fastEthernet 0/1-10
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 10
Switch(config-if-range)#exit

Assign ports f0/1-10 to Vlan10

Switch(config)#interface range fastEthernet 0/11-20
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 20
Switch(config-if-range)#end

Assign ports f0/11-20 to Vlan20

Switch#write

Save

Switch2

Switch>enable
Switch#configure terminal

Enter Global Configuration Mode mode

Switch(config)#vlan 10
Switch(config-vlan)#vlan 20
Switch(config-vlan)#vlan 30
Switch(config-vlan)#vlan 40
Switch(config-vlan)#exit

Create 4 VLANs

Switch(config)#interface range gigabitEthernet 0/2
Switch(config-if)#switchport mode trunk
Switch(config-if)#exit

Define port g0/2 as Trunk mode (* connect to port g0/2 of Switch1)

Switch(config)#interface range fastEthernet 0/1-10,f0/24
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 40
Switch(config-if-range)#exit

Assign ports f0/1-10 and f0/24 to Vlan40

Switch(config)#interface range fastEthernet 0/11-20
Switch(config-if-range)#switchport mode access
Switch(config-if-range)#switchport access vlan 30
Switch(config-if-range)#end

Assign ports f0/11-20 to Vlan30

Switch#write

Save

Test DHCP IP Assignment

Now, we can test the DHCP IP assignment by setting the PCs in each VLAN to obtain IP addresses automatically.

If the configuration is correct, they should receive IP addresses within their respective VLANs’ IP ranges from the DHCP Server.

The IP address assigned to PC1 is 192.168.10.11 (vlan10 network segment)

The IP address assigned to PC2 is 192.168.20.10 (vlan20 network segment)

The IP address assigned to PC3 is 192.168.30.10 (vlan30 network segment)

The IP address assigned to PC0 is 192.168.40.11 (vlan40 network segment)

PC1 Ping PC2, PC3, PC0 (can communicate with each other)

Conclusion:

In this example configuration, the PCs in vlan10, vlan20, vlan30, and vlan40 have successfully obtained IP addresses from the DHCP Server in vlan40. The IP Helper (DHCP Relay Agent) has successfully helped in relaying the DHCP requests and responses between the clients and the DHCP server across different VLANs.