What Is DHCP Snooping And How It Works

The Dynamic Host Configuration Protocol or DHCP is responsible for assigning IP addresses to devices in a network automatically. If there is no DHCP server, users would have to manually configure IP addresses for every client machine and it is impractical in large networks.

But every cloud has a silver lining, hackers can exploit DHCP to launch attacks on devices. They can put a rogue DHCP server in your network which could distributing fake IP addresses leading to network disruption or data theft.

That is where DHCP Snooping comes in. It acts as a security feature on switches to filter and monitor DHCP messages and prevent unauthorized DHCP servers from distributing malicious IP addresses.

In this article, we will break down how DHCP Snooping works, why it is important and how to configure it for network security.

How DHCP Snooping Works

DHCP Snooping is like a security checkpoint for DHCP traffic in a network. It ensures that only trusted DHCP servers can assign IP addresses and block unauthorized ones.

Trusted vs. Untrusted Ports

• Trusted Ports: These are ports where legitimate DHCP servers are connected.
• Untrusted Ports: These are ports where client devices connect. No DHCP server should exist here.

DHCP Snooping Binding Table

The switch maintains a DHCP Snooping Binding Table, which contains:

• MAC Address of the client
• Assigned IP Address
• Lease Time
• VLAN and Interface Details

This table helps in preventing IP spoofing and unauthorized access.

Benefits of DHCP Snooping

Why should you enable DHCP Snooping? Here are its key benefits:

✅ Prevents Rogue DHCP Servers – Blocks unauthorized DHCP servers from assigning fake IP addresses.
✅ Protects Network Integrity – Ensures devices receive correct IP addresses from trusted sources.
✅ Blocks DHCP Spoofing Attacks – Stops hackers from intercepting or redirecting network traffic.
✅ Ensures Network Security – Adds an extra layer of security to prevent unauthorized access.

Configuring DHCP Snooping on a Switch

Want to enable DHCP Snooping? Follow these simple steps:

Basic Commands to Enable DHCP Snooping (Cisco Switch)

• Switch(config)# ip dhcp snooping
• Switch(config)# ip dhcp snooping vlan 10
• Switch(config)# interface gigabitEthernet 1/0/1
• Switch(config-if)# ip dhcp snooping trust
• Switch(config-if)# exit
• Switch(config)# exit

✅ The command ip dhcp snooping trust designates a trusted port where legitimate DHCP servers exist.

Process Flow Chart: DHCP Snooping

1️⃣ Client Sends DHCP Discover
🔽
2️⃣ Switch Receives the DHCP Discover Packet
🔽
3️⃣ Switch Checks the Incoming Port

• 🔹 Trusted Port? ✅ → Forward the request to the DHCP server.
• 🔹 Untrusted Port? ❌ → Inspect the packet.
  🔽
  4️⃣ DHCP Server Sends a DHCP Offer (IP address proposal)
  🔽
  5️⃣ Switch Checks the Source of the DHCP Offer
• 🔹 Comes from a Trusted Port? ✅ → Forward to the client.
• 🔹 Comes from an Untrusted Port? ❌ → Drop the packet (Potential Rogue Server).
  🔽
  6️⃣ Client Sends DHCP Request (Requests the offered IP)
  🔽
  7️⃣ Switch Inspects the DHCP Request Packet
• 🔹 Valid Request? ✅ → Forward to the server.
• 🔹 Suspicious or Spoofed? ❌ → Block and log the attempt.
  🔽
  8️⃣ DHCP Server Sends DHCP Acknowledgment (ACK) (Confirms IP assignment)
  🔽
  9️⃣ Switch Updates the DHCP Snooping Binding Table
• Stores Client MAC, Assigned IP, VLAN, Lease Time, and Interface Details.
  🔽
  🔟 Client Successfully Receives IP Address ✅
• 🚀 Secure network with DHCP Snooping enabled
• ❌ Unauthorized DHCP servers are blocked

Common Issues and Troubleshooting DHCP Snooping

Sometimes, DHCP Snooping might block legitimate traffic due to misconfigurations. Common issues include:

• ❌ Wrong Trusted Port Configuration – Ensure the real DHCP server is on a trusted port.
• ❌ Binding Table Not Updating – Check if the switch is correctly logging DHCP leases.
• ❌ Clients Not Getting IPs – Verify VLAN settings and DHCP relay configurations.

Use the following command to debug DHCP Snooping logs:

Switch# show ip dhcp snooping

Real-World Use Cases of DHCP Snooping

Where is DHCP Snooping used?

• Enterprise Networks – Prevents rogue DHCP servers in large office environments.
• Data Centers – Enhances security in cloud computing infrastructures.
• Public Wi-Fi Networks – Stops unauthorized users from setting up fake DHCP servers.

Read Also Technology Trends in 2025

Conclusion

DHCP Snooping is a critical security feature that ensures DHCP communication remains safe and reliable. By filtering unauthorized DHCP messages, it protects your network from attacks and prevents IP conflicts.

FAQs

1. What happens if DHCP Snooping is not enabled?

Without DHCP Snooping, rogue DHCP servers can distribute malicious IP addresses, leading to network attacks.

2. Can DHCP Snooping block legitimate DHCP servers?

Yes, if the trusted port is not configured properly, the legitimate DHCP server may be blocked.

3. How does DHCP Snooping work with VLANs?

DHCP Snooping can be enabled per VLAN to protect specific network segments.

4. What devices support DHCP Snooping?

Most managed switches, especially Cisco, HP, and Juniper, support DHCP Snooping.

5. Is DHCP Snooping necessary for small networks?

If security is a concern, yes! Even small networks can be vulnerable to rogue DHCP attacks.