About DHCP snooping

DHCP snooping is a security feature for DHCP.

DHCP snooping works between the DHCP client and server, or between the DHCP client and DHCP relay agent. It guarantees that DHCP clients obtain IP addresses from authorized DHCP servers. Also, it records IP-to-MAC bindings of DHCP clients (called DHCP snooping entries) for security purposes.

DHCP snooping defines trusted and untrusted ports to make sure clients obtain IP addresses only from authorized DHCP servers.

• Trusted—A trusted port can forward DHCP messages correctly to make sure the clients get IP addresses from authorized DHCP servers.

• Untrusted—An untrusted port discards received DHCP-ACK and DHCP-OFFER messages to prevent unauthorized servers from assigning IP addresses.

DHCP snooping reads DHCP-ACK messages received from trusted ports and DHCP-REQUEST messages to create DHCP snooping entries. A DHCP snooping entry includes the MAC and IP addresses of a client, the port that connects to the DHCP client, and the VLAN.

The following features need to use DHCP snooping entries:

• ARP attack detection—Uses DHCP snooping entries to filter ARP packets from unauthorized clients. For more information, see Security Configuration Guide.

• MAC-forced forwarding (MFF)—Auto-mode MFF performs the following tasks:

  • Intercepts ARP requests from clients.

  • Uses DHCP snooping entries to find the gateway address.

  • Returns the gateway MAC address to the clients.

  This feature forces the client to send all traffic to the gateway so that the gateway can monitor client traffic to prevent malicious attacks among clients. For more information, see Security Configuration Guide.

• IP source guard—Uses DHCP snooping entries to filter illegal packets on a per-port basis. For more information, see Security Configuration Guide.

• VLAN mapping—Uses DHCP snooping entries to replace service provider VLAN in packets with customer VLAN before sending the packets to clients. For more information, see Layer 2—LAN Switching Configuration Guide.