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Dedicated DHCP & DHCP Relay Agent Lab (Packet Tracer)

Theory prerequisites

Read Switches vs Routers and Subnetting & DHCP. Return to the Lab-Aligned Learning Path after verification.

This lab is designed to take you from DHCP fundamentals to advanced enterprise implementations. You will build and observe:

  • Part 1: Basic Router-Based DHCP Server (Local LAN pool with exclusions and options)
  • Part 2: The DORA Process Deep-Dive (Discover, Offer, Request, Acknowledge in Packet Tracer Simulation Mode)
  • Part 3: Enterprise DHCP Relay Agent (ip helper-address) (Centralized DHCP server providing leases to a remote subnet across a router)

Topology

Client DHCP broadcasts crossing Router0 as relayed unicasts to a centralized DHCP server

1.png

Addressing Plan

Device Interface IP Address Subnet Mask Default Gateway Role
Router0 Gig0/0 192.168.10.1 255.255.255.0 — LAN 1 Gateway (Relay Agent via ip helper-address)
Router0 Gig0/1 192.168.20.1 255.255.255.0 — LAN 2 Gateway (Server Farm)
DHCP-Server0 Fa0 192.168.20.10 255.255.255.0 192.168.20.1 Centralized Dedicated DHCP Server
PC0 Fa0 via DHCP via DHCP 192.168.10.1 Client in LAN 1 (Dynamic Lease: 192.168.10.20+)
PC1 Fa0 via DHCP via DHCP 192.168.10.1 Client in LAN 1 (Dynamic Lease: 192.168.10.21+)

1. Place and Cable Devices

Devices Required

  • 2x Generic PC (PC0, PC1)
  • 2x Switch (2960-24TT: Switch0, Switch1)
  • 1x Router (1941: Router0)
  • 1x Server (DHCP-Server0)

Cabling Connections (Copper Straight-Through)

  • PC0 (FastEthernet0) <—> Switch0 (FastEthernet0/1)
  • PC1 (FastEthernet0) <—> Switch0 (FastEthernet0/2)
  • Switch0 (FastEthernet0/3) <—> Router0 (GigabitEthernet0/0)
  • Router0 (GigabitEthernet0/1) <—> Switch1 (FastEthernet0/1)
  • Switch1 (FastEthernet0/2) <—> DHCP-Server0 (FastEthernet0)

2. Router0 Configuration

Open Router0 CLI and configure the interface IP addresses and the DHCP relay helper:

enable
configure terminal
hostname Router0

! Configure LAN 1 Interface (Client Subnet)
interface GigabitEthernet0/0
 description LAN 1 - Client Network
 ip address 192.168.10.1 255.255.255.0
 ! Forward broadcast DHCP requests to the centralized DHCP server IP
 ip helper-address 192.168.20.10
 no shutdown
exit

! Configure LAN 2 Interface (Server Farm Subnet)
interface GigabitEthernet0/1
 description LAN 2 - Server Farm
 ip address 192.168.20.1 255.255.255.0
 no shutdown
exit

end
write memory

3. Dedicated DHCP-Server0 Configuration

Open DHCP-Server0:

Step A: Configure Static IP on the Server

Go to Desktop > IP Configuration:

  • IP Address: 192.168.20.10
  • Subnet Mask: 255.255.255.0
  • Default Gateway: 192.168.20.1
  • DNS Server: 8.8.8.8

Step B: Configure the DHCP Service for Remote Subnet

Go to Services tab > DHCP:

  • Service: Toggle On
  • Pool Name: LAN1-POOL
  • Default Gateway: 192.168.10.1
  • DNS Server: 8.8.8.8
  • Start IP Address: 192.168.10.20
  • Subnet Mask: 255.255.255.0
  • Maximum number of Users: 50
  • Click Add (or Save)

(Optional: You can also keep the default serverPool for the local 192.168.20.0/24 subnet if you add servers later).

2.png

4. Alternative Implementation: Router as the DHCP Server

If you prefer to run DHCP directly on the Cisco router without a separate server device, configure these commands on Router0 instead of ip helper-address:

! Exclude static infrastructure IPs from being assigned
ip dhcp excluded-address 192.168.10.1 192.168.10.19

! Define DHCP Pool for LAN 1
ip dhcp pool POOL-LAN1
 network 192.168.10.0 255.255.255.0
 default-router 192.168.10.1
 dns-server 8.8.8.8
 domain-name company.local
 lease 7
exit

5. Verification and Testing

A. Test DHCP on Client PCs

  1. Open PC0 > Desktop > IP Configuration and click DHCP.
  2. Verify that PC0 receives:
  3. IP Address: 192.168.10.20
  4. Subnet Mask: 255.255.255.0
  5. Default Gateway: 192.168.10.1
  6. DNS Server: 8.8.8.8
  7. Repeat on PC1; it should receive 192.168.10.21.

B. Command Prompt Verification

On PC0 Command Prompt, test releasing and renewing leases: 

ipconfig /all
ipconfig /release
ipconfig /renew

C. Ping Test Across Subnets

From PC0, ping the DHCP server in LAN 2: 

ping 192.168.20.10

6. Understanding the DORA Process (Simulation Mode Walkthrough)

Switch Packet Tracer to Simulation Mode (Shift + S), set event filters to DHCP and ICMP, and click ipconfig /renew on PC0 to watch each step:

  • 1. Discover (D): PC0 has no IP address, so it broadcasts 255.255.255.255 asking "Is there a DHCP server available?"
  • 2. Relay / Forward: Router0 intercepts the Layer 2 broadcast on Gig0/0, wraps it in a unicast packet, and forwards it to 192.168.20.10 via ip helper-address.
  • 3. Offer (O): DHCP-Server0 replies with an available IP offer (192.168.10.20), default gateway, and lease parameters.
  • 4. Request (R): PC0 formally broadcasts: "I accept the offer of 192.168.10.20 from DHCP-Server0."
  • 5. Acknowledge (A): The DHCP server sends the final ACK confirmation, committing the lease to its database.

7. Key Troubleshooting & Observation Matrix

Issue / Command Cause & Solution Concept Explained
PC gets 169.254.x.x (APIPA) DHCP request timed out; check ip helper-address on Router0 or verify DHCP service is toggled On. APIPA is assigned automatically by the OS when DHCP fails.
show ip dhcp binding (on Router) Displays all active MAC-to-IP lease assignments. Confirms which host has claimed which address.
show ip dhcp pool (on Router) Shows total addresses, leased count, and remaining available IPs. Useful for monitoring pool exhaustion.
Why ip helper-address is required Routers drop Layer 2 broadcasts by default. The helper converts UDP broadcasts (ports 67/68) into unicast packets across subnets. DHCP Relay enables one central DHCP server to manage hundreds of branch subnets.