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