DHCP Snooping & Dynamic ARP Inspection (DAI) Explained – Cisco Layer 2 Security
Learn how DHCP Snooping and Dynamic ARP Inspection (DAI) secure Cisco networks by preventing rogue DHCP servers and ARP spoofing at Layer 2 with real-world examples.
Raza Rehman
🔹 Introduction
Even the most advanced firewalls can’t protect you from a rogue DHCP server or ARP spoofing attack if they start inside your LAN. That’s why Cisco’s DHCP Snooping and Dynamic ARP Inspection (DAI) are essential tools for Layer 2 security.
These two features work together to validate traffic and ensure devices on your network communicate safely and legitimately.
🔹 1. DHCP Snooping – Your Defense Against Rogue DHCP Servers
DHCP Snooping is a Cisco switch feature that classifies ports as trusted or untrusted to prevent malicious or unauthorized DHCP servers from assigning IP addresses.
🧠 Key Concept:
- Trusted Ports: Connect to legitimate DHCP servers or uplinks.
- Untrusted Ports: Connect to end-user devices (clients).
When DHCP Snooping is enabled, the switch creates a DHCP Snooping Binding Table, recording:
- MAC Address
- IP Address
- VLAN
- Lease Time
- Interface
This table becomes the foundation for validating future packets — including ARP inspections.
🔹 2. DHCP Snooping Configuration Example
Switch(config)# ip dhcp snooping
Switch(config)# ip dhcp snooping vlan 10,20
Switch(config)# interface gigabitEthernet0/1
Switch(config-if)# ip dhcp snooping trust
Switch(config)# interface range fa0/1 - 24
Switch(config-if-range)# ip dhcp snooping limit rate 10
✅ Explanation:
- Enables DHCP Snooping globally and for VLANs 10 & 20.
- Marks uplink port G0/1 as trusted.
- Limits DHCP request rate to 10 packets/sec on access ports.
🧩 Pro Tip: Always trust uplinks to DHCP servers or core routers, not user-facing ports.
🔹 3. Verify DHCP Snooping
show ip dhcp snooping
show ip dhcp snooping binding
Sample Output:
MacAddress IpAddress Lease(sec) Type VLAN Interface
00:1A:2B:3C:4D:5E 192.168.10.5 68400 dhcp 10 Fa0/5
🔹 4. Dynamic ARP Inspection (DAI) – Preventing ARP Spoofing
Dynamic ARP Inspection (DAI) works hand-in-hand with DHCP Snooping. It uses the binding table to validate ARP packets on the network.
When a device sends an ARP packet:
- The switch checks if the MAC and IP address pair exists in the binding table.
- If not, the ARP packet is dropped.
This prevents attackers from poisoning the ARP cache or redirecting traffic (a common man-in-the-middle tactic).
🔹 5. DAI Configuration Example
Switch(config)# ip arp inspection vlan 10,20
Switch(config)# interface gigabitEthernet0/1
Switch(config-if)# ip arp inspection trust
Switch(config)# interface range fa0/1 - 24
Switch(config-if-range)# ip arp inspection limit rate 15
✅ Explanation:
- Enables ARP inspection for VLANs 10 & 20.
- Marks uplink G0/1 as trusted.
- Limits ARP traffic to prevent flooding on access ports.
🔹 6. Verify DAI Status
show ip arp inspection
show ip arp inspection statistics
Sample Output:
Vlan Configuration:
VLAN 10: Enabled, 15 pps limit, Logging: Enabled
VLAN 20: Enabled, 15 pps limit, Logging: Enabled
🧠 Tip: If a valid host is being blocked, verify it’s properly registered in the DHCP binding table.
🔹 7. Integration Between DHCP Snooping & DAI
| Feature | Function | Depends On |
|---|---|---|
| DHCP Snooping | Prevents rogue DHCP servers | Standalone |
| DAI | Validates ARP packets | Uses DHCP Snooping binding table |
These two features are strongest together — DHCP Snooping protects IP assignment, while DAI secures MAC/IP identity.
🔹 8. Troubleshooting Commands
| Command | Purpose |
|---|---|
show ip dhcp snooping | Displays DHCP Snooping status |
show ip dhcp snooping database | Checks stored bindings |
show ip arp inspection vlan | Displays ARP inspection configuration |
show ip arp inspection statistics | Shows dropped/validated packets |
debug ip dhcp snooping events | Real-time event log |
debug ip arp inspection | Real-time ARP validation logs |
🔹 9. Common Issues & Fixes
| Issue | Cause | Solution |
|---|---|---|
| Clients not receiving IP | Trusted ports not configured | Configure ip dhcp snooping trust on uplinks |
| Valid hosts blocked | Missing binding entry | Clear & rebuild binding table |
| High CPU load | Flooded DHCP/ARP packets | Limit rate per interface |
| Logs show dropped packets | Untrusted host sending ARP replies | Verify host legitimacy or trust interface |
🔹 10. Security Best Practices
✅ Always enable DHCP Snooping before DAI. ✅ Use rate limiting to prevent flooding. ✅ Trust only uplinks or server-facing interfaces. ✅ Combine with Port Security and BPDU Guard for full L2 protection. ✅ Regularly review binding tables and logs for anomalies.
🔹 Conclusion
Layer 2 attacks are silent but dangerous — they target the foundation of your network. By enabling DHCP Snooping and Dynamic ARP Inspection, Cisco engineers create a secure access layer that automatically detects, blocks, and logs malicious behavior in real-time.
At Netvorx Pro Pvt Ltd, we deploy these protections across enterprise networks to ensure every packet that flows through is both legitimate and trusted.
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