Dynamic ARP Inspection (DAI) is a security feature implemented on Cisco switches that validates Address Resolution Protocol (ARP) packets within a network. ARP is used to map IP addresses to MAC addresses, but it is inherently insecure because it trusts all ARP responses, making networks vulnerable…Dynamic ARP Inspection (DAI) is a security feature implemented on Cisco switches that validates Address Resolution Protocol (ARP) packets within a network. ARP is used to map IP addresses to MAC addresses, but it is inherently insecure because it trusts all ARP responses, making networks vulnerable to ARP spoofing or ARP poisoning attacks.
In an ARP spoofing attack, a malicious actor sends falsified ARP messages over a local network, linking their MAC address with the IP address of a legitimate host. This enables the attacker to intercept, modify, or stop data in transit, potentially leading to man-in-the-middle attacks or denial of service conditions.
DAI works by intercepting all ARP requests and responses on untrusted ports and validating them against a trusted database before forwarding them. This database is typically the DHCP snooping binding table, which contains IP-to-MAC address mappings learned from DHCP transactions. When a switch receives an ARP packet on an untrusted port, it compares the source MAC and IP address information against the binding table. If the information matches, the packet is forwarded normally. If there is no match, the packet is dropped and logged.
Ports connected to other switches or routers are typically configured as trusted ports, meaning ARP packets received on these interfaces bypass DAI validation. User-facing ports are configured as untrusted.
DAI can also perform additional validation checks including source MAC validation, destination MAC validation, and IP address validation to ensure consistency within ARP packets.
To implement DAI effectively, DHCP snooping must first be enabled on the network. For hosts with static IP addresses, ARP access control lists can be configured to permit their traffic.
DAI is an essential layer 2 security mechanism that helps maintain network integrity by ensuring only valid ARP communications occur within the switching infrastructure.
Dynamic ARP Inspection (DAI) - Complete Guide
Why Dynamic ARP Inspection is Important
Dynamic ARP Inspection (DAI) is a critical security feature that protects networks from ARP spoofing and ARP poisoning attacks. These attacks can allow malicious actors to intercept network traffic, perform man-in-the-middle attacks, and redirect data to unauthorized destinations. Understanding DAI is essential for the CCNA exam and for securing enterprise networks.
What is Dynamic ARP Inspection?
DAI is a security feature available on Cisco switches that validates ARP packets in a network. It intercepts, logs, and discards ARP packets with invalid IP-to-MAC address bindings. DAI works by examining ARP requests and responses against a trusted database, typically the DHCP snooping binding table.
How Dynamic ARP Inspection Works
1. Trust States: Ports are classified as either trusted or untrusted. By default, all ports are untrusted when DAI is enabled.
2. Trusted Ports: ARP packets received on trusted ports bypass DAI inspection. These ports typically connect to other switches, routers, or DHCP servers.
3. Untrusted Ports: ARP packets on untrusted ports are validated against the DHCP snooping binding database. Only packets with valid IP-to-MAC bindings are forwarded.
4. DHCP Snooping Dependency: DAI relies on the DHCP snooping binding table to verify ARP packets. DHCP snooping must be enabled for DAI to function properly with dynamic addresses.
5. ARP ACLs: For devices with static IP addresses, ARP access control lists can be configured to permit specific IP-to-MAC bindings.
Configuration Commands
Enable DAI on a VLAN: ip arp inspection vlan
Configure trusted port: interface ip arp inspection trust
Configure ARP ACL for static entries: arp access-list permit ip host mac host
Key Features of DAI
- Rate limiting to prevent DoS attacks on the switch CPU - Logging of invalid ARP packets for security monitoring - Optional validation of source MAC, destination MAC, and IP addresses - Integration with DHCP snooping for comprehensive Layer 2 security
Exam Tips: Answering Questions on Dynamic ARP Inspection
1. Remember the Prerequisites: DAI requires DHCP snooping to be enabled first. If a question mentions DAI is not working, check if DHCP snooping is configured.
2. Default Trust State: All ports are untrusted by default when DAI is enabled. Uplinks to other switches and routers should be configured as trusted.
3. VLAN-Based Feature: DAI is enabled per VLAN, not globally or per interface. Look for the command ip arp inspection vlan in correct answers.
4. Static IP Scenarios: When questions mention devices with static IPs, remember that ARP ACLs are needed since these devices will not have entries in the DHCP snooping binding table.
5. Attack Prevention: Questions about preventing ARP spoofing, ARP poisoning, or man-in-the-middle attacks should point you toward DAI as the solution.
6. Layer 2 Security Stack: DAI is often tested alongside DHCP snooping and port security. Understand how these features complement each other.
7. Error Symptoms: If legitimate hosts cannot communicate after DAI is enabled, consider whether trust states are configured correctly or if ARP ACLs are needed for static devices.
8. Rate Limiting: Be aware that DAI includes rate limiting to protect the switch from being overwhelmed by ARP packets, which could indicate a DoS attempt.