IP Helper

IP address assignment can be static or dynamic. Statically assigned IP address is manually configured by the system administrator, whereas a dynamic IP address also called DHCP automatically configured via a DHCP server.

DHCP allows individual computers to request their IP addresses from a DHCP server, which has been configured to lease addresses to clients. A DHCP malfunction will lead to clients APIPA address auto assignment, and therefore network communication failure.

In order to obtain a lease, the interaction between client and the server has to take place. A series of UDP Broadcast messages exchange are:

• DHCP Discover. Broadcast message set when the client is connected for the first time
• DHCP Offer. The server has received the DHCP Discover from the client, it then replies with an IP address offer. The offer has additional configurations such as the default gateway, the subnet mask, the DNS server, etc...
• DHCP Request. The client has checked the Offer, that’s good, then sends a request “Yes fine by me, please send me this address”.
• DHCP ACK. This message acknowledges the client’s request, “OK, you can use this address”. At this point the client can join the network and exchange information.

There are other DHCP messages such as DHCP Nack, or DHCP Decline, etc… please visit Microsoft TechNet for more information.
IP Helper

There are times when client computers are not in the same subnet as the DHCP server. In this situation, some clients might talk the server, thanks to the IP helper to make it possible. The IP helper feature allows layer 3 switches or router to relay clients DHCP requests to remote servers.

The router or the switch will receive broadcast messages (UDP service) from clients, and then forward them through unicast to the server’s address initially configured.
IP helper configuration is very efficient and fun. In my experience, it has been one of the most amazing commands I have used so far. I used it for the first time when I was configuring VLANs for a local school…

In order to successfully implement IP Helper, below is a basic recommendation guide:

• Make sure the client computer IP settings is configures to acquire its IP address via DHCP
• Make sure the server has the correct address pool configured
• Make sure the address pools have enough IP addresses for the clients
• Make sure the pools are configures with the clients subnets and/or default gateway
• Make sure the IP helper is configured on the router

IP helper configuration

A cisco router ip helper command as below

Router(config-if)#ip helper-address [server IP Address here]
Router(config-if)#exit

A Cisco switch ip helper command

Switch(config-if)#no switchport ##no switchport command first raise the port from layer 2 to layer 3 port
Switch(config-if)#ip help
Switch(config-if)#ip helper-address [DHCP server ip address here]

More about IP helper configuration