📘 CCNA 200-301 v1.1

4.6 Configure and Verify DHCP Client and Relay

1. What is DHCP?

• DHCP stands for Dynamic Host Configuration Protocol.
• Its main job is to automatically assign IP addresses to devices on a network so they can communicate.
• Without DHCP, every device would need a manual IP configuration (which is slow and error-prone).

Think of DHCP as an automatic address manager in a network.

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2. DHCP Client

• A DHCP client is any device that requests an IP address from a DHCP server.
• Examples: computers, laptops, routers, switches (if configured as clients).

How it works (step by step)

1. DHCP Discover:
   • The client doesn’t have an IP yet. It broadcasts a message asking if there’s a DHCP server on the network.
2. DHCP Offer:
   • Any DHCP server that receives the discover message offers an IP address to the client.
3. DHCP Request:
   • The client chooses one offer and asks the server to assign it that IP.
4. DHCP Acknowledgment (ACK):
   • The server confirms the assignment and sends details like:
     • IP address
     • Subnet mask
     • Default gateway
     • DNS server

This whole process is called the DORA process: Discover → Offer → Request → Acknowledge.

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3. Configuring a DHCP Client on a Cisco Device

On routers or switches, you can set an interface to request an IP from a DHCP server.

Command example:

interface GigabitEthernet0/0
 ip address dhcp
 no shutdown

• ip address dhcp → Makes the interface a DHCP client.
• no shutdown → Turns the interface on.

Verify DHCP client assignment:

show ip interface brief
show dhcp lease

These commands confirm the IP assigned to the interface by DHCP.

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4. DHCP Relay

• Sometimes, a DHCP server is not on the same network as the client.
• Routers do not forward broadcasts by default, so a DHCP request cannot reach a server on a different network.
• DHCP Relay solves this problem.

Function:

• Receives DHCP requests from clients.
• Forwards them to a remote DHCP server using unicast.
• Sends the server’s response back to the client.

When is DHCP relay used?

• Large networks with multiple subnets where one DHCP server serves many networks.

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5. Configuring DHCP Relay on a Cisco Device

• Use the command ip helper-address on the router interface facing the client subnet.
• Example:

interface GigabitEthernet0/1
 ip address 192.168.10.1 255.255.255.0
 ip helper-address 192.168.1.100
 no shutdown

• 192.168.10.1 → Router interface in client’s subnet.
• ip helper-address 192.168.1.100 → IP of the remote DHCP server.

Verify DHCP relay:

show ip dhcp binding
show running-config

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6. Key Exam Points to Remember

1. DHCP Client:
   • Requests an IP from DHCP server automatically.
   • Use ip address dhcp to configure on Cisco interfaces.
   • Verify using show ip interface brief and show dhcp lease.
2. DHCP Relay:
   • Forwards client requests to a DHCP server on another subnet.
   • Configured with ip helper-address on the router interface.
   • Verify with show running-config or show ip dhcp binding.
3. DORA Process: Discover → Offer → Request → Acknowledge.
4. DHCP relay only forwards certain UDP ports:
   • Port 67 for server, 68 for client (standard DHCP ports).

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✅ Summary Table:

Feature	Purpose	Cisco Command	Verification
DHCP Client	Auto-assign IP to interface	ip address dhcp	show ip interface brief
DHCP Relay	Forward DHCP requests to remote server	ip helper-address <server-IP>	show running-config

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