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DHCP IP Helper address in Cisco devices

As a CCNA / CCNP candidate you are expected to understand the purpose and function of the IP Helper address .

The CCNA / CCNP’s exam will ask of you to determine the purpose of the IP Helper command, what it contains and the role it plays in your network.

The purpose of the IP helper command for the purposes of DHCP (Dynamic Host Configuration Protocol) is to assist a host within a broadcast domain to acquire an IP address from a DHCP server that resides within another broadcast domain.

Before we take a detailed view of how the IP helper command works we will firstly need to look at what defines a broadcast domain. A broad is a message which is generated by host or hosts when they wish to either announce their presence to the other hosts on the network or they need to discover some entity on the network. The two most common types of broadcast messages that are issued from a host are ARP (Address Resolution Protocol) messages to resolve a known IP address to a Layer 2 MAC address or a DHCP discover message.

Both the ARP message and the DHCP discover message will received by all of the host within the VLAN or Subnet of the host that generated the broadcast message, if the broadcast message is received by a router interface the default action of the router interface is to discard the message unless it itself is the intended recipient of the broadcast, say for example the generating host was ARP’ing for it’s default gateways’ MAC address.

The router will not by default forward the broadcast of any nature across to another subnet. Imagine if the router did this and forwarded all broadcasts from one network into another network, your router at home or at the office is connected to the internet, imagine if your router as to forward by default your broadcasts from one subnet, i.e. yours over into the public network, and multiplied that by a few million times one for every router that is connected to the internet, well that would be a lot of traffic, so again by default it is not in the interest of performance that broadcasts are forwarded from one subnet to another.

So where does that leave us in regards to the client sending out DHCP Discover messages in the attempt of getting a valid host IP address. If no DHCP server resides within the same subnet that the client resides in because the DHCP server lives in another subnet in then there is no chance of the client getting an IP address.

We in this case have to assist the client in getting an IP address from the DHCP server which lives in another network and to do this we need to use the “IP helper-address” command which is placed on the same interface that is the default gateway for the host requesting the IP address. The command would be as follows

Router(config-if)#ip helper-address x.x.x.x

The x.x.x.x would represent the address of the DHCP server. It is important that the router knows how to reach the subnet that the DHCP resides within, unless this is the case the router will never be able to forward the DHCP discover messages which are generated by the client to the DHCP server.

When the routers’ interface receives the DHCP discover message from the client it will relay the DHCP discover message over to the DHCP server placing it’s own interface IP address in a field called the “Relay agent”, this is done so that the DHCP server knows from which of its address scopes to draw a valid address from. It is important to bear in mind at this point that the original DHCP discover message which is a broadcast is relayed over to the DHCP server as a unicast message.