IP Helper Address Utilization

By Edward Tetz from Cisco Networking All-in-One For Dummies

You can implement an IP Helper address to forward Dynamic Host Configuration Protocol (DHCP) broadcasts on to their appropriate destination. In this way, you can implement a single DHCP server as one management point where you can check the leases for any device on your network and manage all the IP subnets on your network.

Each subnet that does not have its own DHCP server will be configured with an ip helper-address command. The configuration for this command follows; note that you first must change the router interface to which you will be assigning the helper.

Router2>enable
Password:
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface fastEthernet 0/1
Router2(config-if)#ip helper-address global 192.168.1.8
Router2(config-if)#exit
Router2(config)#exit

When you enable the IP Helper address, all traffic for the UDP ports are automatically forwarded to the address specified.

UDP Traffic Forwarded by IP Helper Address
UDP PORT	Common Name
69	TFTP
67	BOOTP Client
68	BOOTP Server
37	Time Protocol
49	TACACS
53	DNS
137	NetBios
138	NetBios Datagram

To restrict the forwarded traffic, you can specify the restrictions by adding any of the following no commands to your configuration at the Global Configuration mode.

Router2>enable
Password:
Router2#configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#no ip forward-protocol udp 37
Router2(config)#no ip forward-protocol udp 49
Router2(config)#no ip forward-protocol udp 53
Router2(config)#no ip forward-protocol udp 67
Router2(config)#no ip forward-protocol udp 68
Router2(config)#no ip forward-protocol udp 137
Router2(config)#no ip forward-protocol udp 138
Router2(config)#exit