from the net : DHCP Snooping

The article describes DHCP Snooping as a security feature on Cisco switches that protects against DHCP-based threats by filtering out untrusted DHCP messages and maintaining a DHCP snooping binding table. This binding table records details such as MAC address, IP address, lease time, binding type, VLAN, and interface—but only for devices connected via untrusted interfaces; trusted interfaces (typically toward the DHCP server or backbone) are not included. By acting like a firewall between untrusted hosts and DHCP servers, the switch can block unauthorized or malicious DHCP traffic and clearly differentiate between trusted and untrusted parts of the network.

How it functions in the network and its broader utility:

DHCP Snooping is enabled per switch and per VLAN. Once activated on a switch, it operates as a Layer 2 bridge that inspects DHCP traffic within VLAN domains. This helps prevent common attacks—especially Man-in-the-Middle attacks—where a rogue device responds to DHCPDISCOVER requests before the legitimate DHCP server does. By marking the DHCP server port as trusted (even if it's a trunk), and marking all other user-facing ports as untrusted, the switch ensures only valid DHCP replies get forwarded.

Integration and configuration considerations:

DHCP Snooping not only protects client-server communication but also supports other security features such as IP Source Guard (IPSG) and Dynamic ARP Inspection (DAI) by supplying validated binding data. When configuring, global enablement on the switch is required before applying it to specific VLANs. Additionally, enabling DHCP Snooping automatically disables any DHCP relay-related configuration commands such as ip dhcp relay information check, policy, trusted, and trust-all.

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