DHCP snooping is supported on Layer 2 and Layer 3 traffic. DHCP snooping on Layer 2 interfaces does require a relay agent.Binding table entries are deleted when a lease expires or when the relay agent encounters a DHCPRELEASE. Line cards maintain a list ofsnooped VLANs. When the binding table is exhausted, DHCP packets are dropped on snooped VLANs, while these packets are forwardedacross non-snooped VLANs. Because DHCP packets are dropped, no new IP address assignments are made. However, DHCPRELEASEand DHCPDECLINE packets are allowed so that the DHCP snooping table can decrease in size. After the table usage falls below themaximum limit of 4000 entries, new IP address assignments are allowed.NOTE: DHCP server packets are dropped on all not trusted interfaces of a system configured for DHCP snooping. To preventthese packets from being dropped, configure ip dhcp snooping trust on the server-connected port.Enabling DHCP SnoopingTo enable DHCP snooping, use the following commands.1 Enable DHCP snooping globally.CONFIGURATION modeip dhcp snooping2 Specify ports connected to DHCP servers as trusted.INTERFACE modeINTERFACE PORT EXTENDER modeip dhcp snooping trust3 Enable DHCP snooping on a VLAN.CONFIGURATION modeip dhcp snooping vlan nameEnabling IPv6 DHCP SnoopingTo enable IPv6 DHCP snooping, use the following commands.1 Enable IPv6 DHCP snooping globally.CONFIGURATION modeipv6 dhcp snooping2 Specify ports connected to IPv6 DHCP servers as trusted.INTERFACE modeipv6 dhcp snooping trust3 Enable IPv6 DHCP snooping on a VLAN or range of VLANs.CONFIGURATION modeipv6 dhcp snooping vlan vlan-idAdding a Static Entry in the Binding TableTo add a static entry in the binding table, use the following command.• Add a static entry in the binding table.EXEC Privilege mode284 Dynamic Host Configuration Protocol (DHCP)
