DHCP snooping is a Cisco Catalyst feature that determines which switch ports can respond to DHCP requests. Ports are identified as trusted and untrusted. Trusted ports can source all DHCP messages; untrusted ports can source requests only. Trusted ports host a DHCP server or can be an uplink toward the DHCP server. If a rogue device on an untrusted port attempts to send a DHCP response packet into the network, the port is shut down. This feature can be coupled with DHCP options in which switch information, such as the port ID of the DHCP request, can be inserted into the DHCP request packet.

As shown in Figures 1 and 2, untrusted ports are those not explicitly configured as trusted. A DHCP binding table is built for untrusted ports. Each entry contains a client MAC address, IP address, lease time, binding type, VLAN number, and port ID recorded as clients make DHCP requests. The table is then used to filter subsequent DHCP traffic. From a DHCP snooping perspective, untrusted access ports should not send any DHCP server responses.

These steps illustrate how to configure DHCP snooping on a Catalyst 2960 switch:

Step 1. Enable DHCP snooping using the ip dhcp snooping global configuration mode command.

Step 2. Enable DHCP snooping for specific VLANs using the ip dhcp snooping vlan number command.

Step 3. Define ports as trusted at the interface level by defining the trusted ports using the ip dhcp snooping trust command.

Step 4. (Optional) Limit the rate at which an attacker can continually send bogus DHCP requests through untrusted ports to the DHCP server using the ip dhcp snooping limit rate rate command.