For implementation design above. Plese adding helper-address (ip dhcp server) on interface router.

Router

Router(config)#interface fastethernet 0/0

 

Moves to interface configuration mode.

Router(config-if)#ip helper-address 172.16.20.2

 

DHCP broadcasts will be forwarded as a unicast to this specific address rather than be dropped by the router.

Layer 3 Switch

Switch(config)#interface vlan 10

àMoves to SVI configuration mode.

Switch(config-if)#ip helper-address 172.16.20.2

àDHCP broadcasts will be forwarded as a unicast to this specific address rather than be dropped by the router.

NOTE

  • The ip helper-address command forwards broadcast packets as a unicast to eight different UDP ports by default:
  • TFTP (port 69)
  • DNS (port 53)
  • Time service (port 37)
  • NetBIOS name server (port 137)
  • NetBIOS datagram server (port 138)
  • Boot Protocol (BOOTP) client and server datagrams (ports 67 and 68)
  • TACACS service (port 49)
  • Host Name Service (port 42)

To close some of these ports, use the no ip forward-protocol udp x command at the global configuration prompt, where x is the port number you want to close. The following command stops the forwarding of broadcasts to port 49:

Router(config)#no ip forward-protocol udp 49

To open other UDP ports, use the ip forward-helper udp x command, where x is the port number you want to open:

Router(config)#ip forward-protocol udp 517