Configure DHCP Snooping

 Enable DHCP snooping globally
 S1(config)# ip dhcp snooping 
 S1(config)# ip dhcp snooping information option 
 
 Enable DHCP snooping for VLAN 1 and 20
 S1(config)# ip dhcp snooping vlan 1,20 
 
 Limit the number of DHCP requests on an interface
 S1(config-if)# ip dhcp snooping limit rate 10
 
 Identify the trusted interface
 S1(config)# interface f0/5 
 S1(config-if)# description connects to DHCP server 
 S1(config-if)# ip dhcp snooping trust

 Verify 
 S1# show ip dhcp snooping