Before start-up I want to give some basic information about DHCP. As you know DHCP use UDP 67 and UDP 68 ports. It has a messaging system for the communication between DHCP Server and DHCP Client. These messaging system’s messages and their types are mentined below:
• DHCP Discover (broadcast)
• DHCP Offer(broadcast)
• DHCP Request (broadcast)
• DHCP Ack (broadcast)
• DHCP Nak (unicast)
• DHCP Release (unicast)
• DHCP Decline (unicast)
• DHCP Inform (unicast)
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Firstly, a client sends a broadcast “DHCP Discovery” message that mentions that it need an ip address.
Then, the DHCP servers reply with configuration offers to the client by “DHCP Offer” unicast message.
After that DHCP client sends a broadcast “DHCP Request” message to the network with the “Transaction ID” of the first DHCP Server that send Offer. The other servers understand that client wants to use the server that has the related “Transaction ID”.
Lastly, the Server sends a unicast “Acknowledgement” message to the client that mentions the ip assignment is successfully done or it send a refuse messaged named “DHCP-NACK”.
To configure a Packet Tracer Router ’s DHCP, we must follow some basic steps. For this configuration the important point is broadcast domains. If we have only one broadcast domain in our topology, our work is simpler, else we must get help from “ip-helper address” command.
What is ip helper address command? Ip helper address command is the command that helps us to convince the router and make it pass the broadcast packets.
Now, let’s go to our two different configuration topology and see how to configure a server in packet tracer for DHCP, how to configure a DHCP Server in packet tracer.
For One Broadcast Domain
Our one broadcast domain topology is like below. There is a router that will carry our DHCP server role beside its routing functionalities. And there is a switch for PCs.
On routerA, firstly we will give an ip address to the router interface that is connected to the switch.Secondly that we will create a DHCP pool named IPD. In this pool we will mention ip addresses that will be given to the DHCP clients.After that we will assign the router’s interface address as a default-router address for clients. And in the last part, we will exclude some addresses with “ip dhcp excluded address” command, that we don’t want to use during this dynamic ip assignments. With “ip dhcp excluded address” command, the mentined addresses will not used in the pool.
RouterA# config terminal
After this configuration, when we check the ip address of PC0, we will see the ip address 192.168.10.11 . Because it is the first available address in DHCP pool.
Our second case is how to enable DHCP on router for multiple broadcast domains. In our second Cisco packet tracer example, we will use ip helper, cisco command “ip helper-address“. So, what is ip helper address?
Many CCNAs learn that routers do not pass broadcasts. But progress in CCIE, network engineers learn that it is not true. Because you can pass broadcast traffic for many protocols as DHCP by “ip-helper address” command. Here we will refer only the broadcast of DHCP requests. We can use a router as a DHCP Server again, but I use a separate DHCP Server instead of router in this topology.
In the first place we will configure the DHCP Server for its DHCP pools and its ip configuration. The ip address is 192.168.1.10 and the default gateway will be the routers interface’s ip address that is face to DHCP server.
For the subnets 192.168.1.0 and 10.10.0.0 there must be two DHCP pool.The below screenshot is showing how these assignments will be done in DHCP Server.
In the Packet Tracer router the following configuration will be done for two different subnet DHCP achivement:
RouterC # config terminal
After this configuration, we can try to dynamic ip assignment on PC by selecting the dynamic option on ip configuration screen like below.