The Juniper EX switch must be configured to enable Dynamic Address Resolution Protocol (ARP) Inspection (DAI) on all user VLANs.

Severity	Group ID	Group Title	Version	Rule ID	Date	STIG Version
medium	V-253961	SRG-NET-000362-L2S-000027	JUEX-L2-000140	SV-253961r1168045_rule	2025-12-10	2

Description
DAI intercepts ARP requests and verifies that each of these packets has a valid IP-to-MAC address binding before updating the local ARP cache and before forwarding the packet to the appropriate destination. Invalid ARP packets are dropped and logged. DAI determines the validity of an ARP packet based on valid IP-to-MAC address bindings stored in the DHCP snooping binding database. If the ARP packet is received on a trusted interface, the switch forwards the packet without any checks. On untrusted interfaces, the switch forwards the packet only if it is valid. This requirement is applicable to VLANs with active access interfaces assigned but not to switches that only trunk the VLANs between switches.

Description

DAI intercepts ARP requests and verifies that each of these packets has a valid IP-to-MAC address binding before updating the local ARP cache and before forwarding the packet to the appropriate destination. Invalid ARP packets are dropped and logged. DAI determines the validity of an ARP packet based on valid IP-to-MAC address bindings stored in the DHCP snooping binding database. If the ARP packet is received on a trusted interface, the switch forwards the packet without any checks. On untrusted interfaces, the switch forwards the packet only if it is valid. This requirement is applicable to VLANs with active access interfaces assigned but not to switches that only trunk the VLANs between switches.

ℹ️ Check
Review the switch configuration to verify that Dynamic ARP DAI feature is enabled on all user VLANs. Configuring DAI automatically enables DHCP snooping. Devices like printers, servers, and VoIP phones are under enterprise control and connected to controlled access interfaces (802.1x, Static MAC Bypass, or MAC RADIUS), making them trusted sources in non-user-facing VLANs. Verify DAI on user-facing or untrusted VLANs. [edit vlans] <untrusted VLAN name> { vlan-id <VLAN ID>; forwarding-options { dhcp-security { arp-inspection; } } } Note: DAI depends upon DHCP snooping or static MAC address bindings. If DAI is not enabled on all user VLANs, this is a finding.

ℹ️ Check

Review the switch configuration to verify that Dynamic ARP DAI feature is enabled on all user VLANs. Configuring DAI automatically enables DHCP snooping. Devices like printers, servers, and VoIP phones are under enterprise control and connected to controlled access interfaces (802.1x, Static MAC Bypass, or MAC RADIUS), making them trusted sources in non-user-facing VLANs. Verify DAI on user-facing or untrusted VLANs. [edit vlans] <untrusted VLAN name> { vlan-id <VLAN ID>; forwarding-options { dhcp-security { arp-inspection; } } } Note: DAI depends upon DHCP snooping or static MAC address bindings. If DAI is not enabled on all user VLANs, this is a finding.

✔️ Fix
Configure the switch to have ARP DAI enabled on all user VLANs. This requirement is applicable to VLANs with active access interfaces assigned but not to switches that only trunk the VLANs between switches. set vlans <untrusted VLAN name> vlan-id <VLAN ID> set vlans <untrusted VLAN name> forwarding-options dhcp-security arp-inspection