Dell C1048P Port Extender Networking Configuration Guide for the C9000 Series - Page 380

packet arrived on the correct port. Packets that do not pass this check are forwarded to the server for validation. This checkpoint prevents an attacker from spoofing a client and declining or releasing the real client's address. Server-originated packets (DHCPOFFER, DHCPACK, and DHCPNACK) that arrive on a not trusted port are also dropped. This checkpoint prevents an attacker from acting as an imposter as a DHCP server to facilitate a man-in-the-middle attack. Binding table entries are deleted when a lease expires, or the relay agent encounters a DHCPRELEASE, DHCPNACK, or DHCPDECLINE. DHCP snooping is supported on Layer 2 and Layer 3 VLANs. DHCP snooping on Layer 3 interfaces depends on the configured DHCP relay agent (ip helper-address). DHCP snooping on Layer 2 interfaces does not require a relay agent. When DHCP snooping VLAN interface is deleted or the operation is down, the corresponding learned entries are also removed from the snooping table. NOTE: DHCP server packets are dropped on all not trusted interfaces of a system configured for DHCP snooping. To prevent these packets from being dropped, configure ip dhcp snooping trust on the server-connected port. Enabling DHCP Snooping To enable DHCP snooping, use the following commands. 1. Enable DHCP snooping globally. CONFIGURATION mode ip dhcp snooping 2. Specify ports connected to DHCP servers as trusted. INTERFACE mode ip dhcp snooping trust 3. Enable DHCP snooping on a VLAN. CONFIGURATION mode ip dhcp snooping vlan name Adding a Static Entry in the Binding Table To add a static entry in the binding table, use the following command. • Add a static entry in the binding table. EXEC mode EXEC Privilege mode ip dhcp snooping binding mac Clearing the Binding Table To clear the binding table, use the following command. • Delete all of the entries in the binding table. EXEC Privilege mode 380 Dynamic Host Configuration Protocol (DHCP)