Dynamic ARP Inspection (DAI)

Note

DAI is supported only in non-DHCP environments.

On VLANs, DAI can examine incoming ARP packets. DAI discards packets with invalid IP address and MAC address bindings, guarding against ARP poisoning. Only valid ARP requests and responses are relayed. You specify valid, static IP address and MAC address bindings in the permit statements of ARP ACLs.

You decide which ports to define as trusted or untrusted. ARP packets on trusted ports bypass all DAI validations and are forwarded as required. DAI examines ARP packets only on untrusted ports.

DAI monitors untrusted ports as follows.

Intercepts ARP requests and responses.
Compares the IP address and MAC address bindings with the permit statements in the ACL applied to the VLAN.
Drops invalid packets.
Forwards valid packets to the appropriate destination.

Table 1. DAI on trusted and untrusted ports
VLAN setting	Port setting	Action
DAI disabled	Trusted or untrusted	All incoming ARP packets are hardware-forwarded.
DAI enabled	Trusted	All incoming ARP packets are trapped to the CPU and then software-forwarded.
DAI enabled	Untrusted	All incoming ARP packets are trapped to the CPU. Following DAI, the packets are software-forwarded or dropped.