Junos' Dynamic ARP Inspection: A Closer Look

In the world of networking, security is paramount. One of the key security features provided by Juniper Networks in their Junos operating system is Dynamic ARP Inspection (DAI). This feature is particularly important for those studying for the JNCIS-ENT certification, as it forms a crucial part of the exam's syllabus. In this blog post, we will delve into the details of DAI, its configuration, and its role in network security.

What is Dynamic ARP Inspection?

Dynamic ARP Inspection is a security feature that prevents ARP spoofing attacks. ARP spoofing is a type of attack where an attacker sends falsified ARP (Address Resolution Protocol) messages over a local area network. This results in the linking of an attacker's MAC address with the IP address of a legitimate computer or server on the network. DAI works by intercepting, logging, and discarding ARP packets with invalid IP-to-MAC address bindings.

How Does DAI Work?

DAI validates ARP packets in a network. It intercepts each ARP request and reply on untrusted ports, and verifies that each of these intercepted packets has a valid IP-to-MAC address binding before updating the local ARP cache or forwarding the packet to the appropriate destination.

DAI uses the DHCP snooping binding database to validate the ARP packets. This database is built when a client sends a DHCP request and the switch forwards the request to the DHCP server. The server replies with a DHCP response that contains the client MAC address, client IP address, lease time, and VLAN number. The switch then creates an entry in the DHCP snooping binding database.

Configuring DAI on Junos

To configure DAI on Junos, you need to first enable DHCP snooping on the VLANs and then enable DAI. Here is a step-by-step guide:

  1. Enable DHCP snooping on the VLAN:
user@switch# set ethernet-switching-options secure-access-port vlan employees dhcp-snooping
  1. Enable DAI on the VLAN:
user@switch# set ethernet-switching-options secure-access-port vlan employees arp-inspection
  1. Commit the configuration:
user@switch# commit

Verifying DAI Configuration

You can verify the DAI configuration by using the show arp-inspection command:

user@switch> show ethernet-switching-options secure-access-port

This command displays the status of DAI and DHCP snooping on all VLANs.


Dynamic ARP Inspection is a powerful security feature that can protect a network from ARP spoofing attacks. It is an essential topic for those studying for the JNCIS-ENT certification. Understanding how DAI works and how to configure it on Junos is crucial for network security and for passing the certification exam.

Remember, DAI is not a standalone solution. It should be used as part of a comprehensive security strategy that includes other features like IP Source Guard and DHCP Snooping. Always ensure that you understand the security needs of your network and configure the appropriate features to meet those needs.

© Ben Jacobson.RSS