Configure and verify DHCP (IOS Router)

Exam: Cisco 200-301 - Cisco Certified Network Associate (CCNA)

The Configure and verify DHCP (IOS Router) is a sub topic under IP services that is a part of the Cisco certified Network Associate Exam. This is just a guideline into what can be asked in this exam from this particular area of knowledge. DHCP is the abbreviation of Dynamic Host Configuration Protocol. In this section you will be tested on your knowledge about how exactly the DHCP works. The DHCP is a protocol that is used to automatically assign IP address over LAN (land area networks). The DHCP server supports only Ethernet interfaces.

Configuring router interfaces to use DHCP

This is the first sub topic under the main topic. Under this you must understand well how you can configure the router to get the interface IP address automatically. The single command that you need to use is

Router(config-if)#ip address dhcp

The DHCP server database is like a tree. The address pool is the root of the tree and the branches are the subnetwork address pools. The domain name must be the last to be configured at the top of the tree. The command used for domain name allocation is - Router(dhcp-config)# domain-name domain .

The command Router(config)# ip dhcp pool name, will a name for the DHCP sever address pool. This will also initiate the DHCP pool configuration.

The next you will have to use the command Router(dhcp-config)# network network-number [mask |/prefix-length] this will specify the subnet network number and also the mask.

The DHCP relay agents are nothing but a host that forwards the DHCP packets from the server to the client and vice versa. The relay agents get the DHCP messages and then they generate new DHCP messages that can be forwarded to another interface. The Cisco IOS DHCP effectively supports a number of relay host interfaces.

DHCP options (Basic overview and functionality)

The CiscoIOS DHCP Server has a number of benefits and they are:

  1. It helps in centralized management. As the DHCP server can maintains many subnets the administrator just needs to update a single and central server when some changes happen.
  2. It reduces the cost and the task of client configuration. The DHCP server is much easy to configure this is exactly why it reduces the operational costs. It can be also deployed by non technical users.
  3. It reduces the internet access cost to a great extent. This is possible mainly because automatic IP address assignment is used at each remote site. The static IP addresses are always more expensive.

There are many benefits that the client also enjoys by using the DHCP server and they are as follows:

  1. It reduces the time that is needed to configure and deploy.
  2. It also allows the customers to control the IP address assigned from a centralised location.
  3. This can also reduce the configuration errors to a good extent.

Before configuring the CiscoIOS DHCP Server one must complete the following tasks:

  1. You must identify the external file transport protocol (FTP), the remote copy protocol (rcp) or the Trivial File Transfer Protocol (TFTP) as these will be used to store the DHCP binding
  2. You must identify the IP address that you will use and also the one that you will exclude.
  3. You must decide the domain name
  4. The following DHCP options must be identified:

Default routers

NetBIOS name server

DNS (domain name servers)

Default boot image name

You must take a decision on the NetBIOS node type to be used.

The following command will help you to configure the database agent this also helps the interval between database updates and database transfers for each agent. This command is used in global configuration mode:

Router(config)# ip dhcp database url [timeout seconds | write-delay seconds]

If you want you can also disable the recording of a DHCP database agent. This is done when the recording is creating a conflict in the server. The command is used in global configuration mode and the command is as follows:

Router(config)# no ip dhcp conflict logging

The following command must be used in the global configuration mode for configuring the DHCP address pool name and also for entering the DHCP pool configuration mode. This is a very important command and you must know how and when to use it.

Router(config)# ip dhcp pool name

We will now move on to how to specify the subnet network number and the mask for the DHCP address pool. The command is as follows and must be used in DHCP pool configuration mode.

Router(dhcp-config)# network network-number [mask |/prefix-length]

The prefix length is gives the number of bits that make up the address prefix.

Excluded addresses

Under this section you must understand that the DHCP server by default assumes that all the IP addresses in the subnet pool are available but that is not true. You must clearly mention which are the IP addresses that it must exclude. The enclosed command can be used to specify the IP addresses that the DHCP server should not assign to the DHCP clients.

Router(config)# ip dhcp excluded-address low-address [high-address]

Lease time

It is a default rule that all IP addresses that are assigned by the DHCP have a lease of one day. For this time the IP address will be valid. If you want to change the lease time you can use the enclosed command.

Router(dhcp-config)# lease {days [hours][minutes] | infinite}

You can also use the “show Ip dhcp binding” to show the time in which the lease will expire. You can also use this command to show the date of the host’s IP address.

We hope this tutorial will help you to prepare better for the CCNA exam and achieve the success you desire to much in your life. Try to prepare these topics and have a detailed knowledge in order to perform better in the exam. The exam is a closed book exam and preparation on these lines can be of great help.

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