V-253960

Discussion

IP Source Guard provides source IP address filtering on an untrusted layer 2 interface to prevent a malicious host from impersonating a legitimate host by assuming the legitimate host's IP address. The feature uses dynamic DHCP snooping and static IP source binding to match IP addresses to hosts on untrusted layer 2 access interfaces. Initially, all IP traffic on the protected access interface is blocked except for DHCP packets. After a client receives an IP address from the DHCP server, or after static IP source binding is configured by the administrator, all traffic with that IP source address is permitted from that client. Traffic from other hosts is denied. This filtering limits a host's ability to attack the network by claiming a neighbor host's IP address.

Check Content

Review the switch configuration to verify that IP Source Guard is enabled on all user-facing or untrusted VLANs with active access interfaces. Configuring IP Source Guard automatically enables DHCP snooping.

Devices such as 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 nonuser-facing VLANs.

Verify IP Source Guard on user-facing or untrusted VLANs with active access interfaces.

user@host> show configuration vlans
<untrusted VLAN name> {
    vlan-id <VLAN ID>;
    forwarding-options {
        dhcp-security {
            ip-source-guard;
        }
    }
}
Note: IP Source Guard depends on DHCP snooping or static MAC address bindings.

This check is not applicable to switches configured with user-facing or untrusted VLANs merely for inclusion in trunks (e.g., core or distribution switch without access interfaces assigned to user-facing or untrusted VLANs).

If the switch does not have IP Source Guard enabled on all user-facing or untrusted VLANs with active access interfaces, this is a finding.