Before we take a detailed view of how the IP helper command works we can first of all need to examine what defines a printed domain. A vast is a message that is generated by means of host or hosts after they want to both announce their presence to the opposite hosts at the network or they want to discover a few entity at the community. The maximum not unusual forms of broadcast messages which might be issued from a host are ARP (Address Resolution Protocol) messages to solve a recognised IP deal with to a Layer 2 MAC address or a DHCP discover message.
Both the ARP message and the DHCP find out message will obtained by way of all of the host in the VLAN or Subnet of the host that generated the broadcast message, if the printed message is obtained with the aid of a router interface the default movement of the router interface is to discard the message except it itself is the supposed recipient of the published, say as an example the producing host became ARP’ing for it is default gateways’ MAC deal with.
The router will not by using default ahead the of any nature across to some other subnet. Imagine if the router did this and forwarded all pronounces 女傭 from one community into some other network, your router at home or at the office is connected to the net, consider if your router as to forward by way of default your proclaims from one subnet, i.E. Yours over into the general public community, and multiplied that by some million instances one for each router this is related to the internet, well that could be lots of visitors, so again by way of default it isn’t always within the interest of overall performance that announces are forwarded from one subnet to some other.
So wherein does that depart us in regards to the consumer sending out DHCP Discover messages in the strive of having a legitimate host IP cope with. If no DHCP server is living in the identical subnet that the customer resides in because the DHCP server lives in any other subnet in then there is no hazard of the consumer getting an IP address.
We in this example should assist the consumer in getting an IP address from the DHCP server which lives in every other community and to do this we need to use the “IP helper-cope with” command which is located on the identical interface this is the default gateway for the host inquiring for the IP address. The command might be as follows
Router(config-if)#ip helper-address x.X.X.X
The x.X.X.X might constitute the cope with of the DHCP server. It is crucial that the router is aware of the way to attain the subnet that the DHCP resides inside, except this is the case the router will by no means be able to ahead the DHCP discover messages which can be generated via the consumer to the DHCP server.
When the routers’ interface receives the DHCP find out message from the customer it will relay the DHCP discover message over to the DHCP server putting it is own interface IP deal with in a discipline referred to as the “Relay agent”, that is carried out so that the DHCP server knows from which of its deal with scopes to draw a legitimate deal with from. It is critical to bear in thoughts at this factor that the original DHCP discover message that’s a broadcast is relayed over to the DHCP server as a unicast message.
Joe Spoto is a senior lecturer at Commsupport networks CCNA within the United Kingdom. Joe teaches Cisco CCNA, CCNP, CCVP publications while he isn’t always out on the street fixing and building networks, if you want to find out greater about what we do at Commsupport please visit us at CCNA Commsupport run free one day training periods and free online webinars, CCNP