CCNP BSCI Official Exam Certification Guide, Fourth Edition

386 Chapter 13: Dynamic Host Control Protocol

DHCP Roles

There are at least two devices involved in any DHCP setup—a DHCP server and a DHCP

client. There are often additional devices in the network between the client and server, called

DHCP relays. The following sections describe the function and conﬁguration of each of

these DHCP roles.

Acting as a DHCP Server

The following steps conﬁgure an IOS device as a DHCP server:

Step 1 Create a pool of addresses to assign to clients. The syntax for this command is

Router(config)# ip dhcp pool

name

Step 2 Assign a network to the pool:

Router(config-dhcp)# network

network/mask

Step 3 Tell the client how long it can keep the address—this is called the lease

period. Most DHCP implementations use a three-day lease, but IOS

defaults to one day:

Router(config-dhcp)# lease

days

Step 4 Identify the DNS server:

Router(config-dhcp)# dns-server

address

Step 5 Finally, identify the default gateway:

Router(config-dhcp)# default-router

ip-address

NOTE Many client operating systems use something called Automatic Private IP Addressing.

This process assigns an IP address, even in the absence of a DHCP server. If a DISCOVER

message is not answered, the client picks a random 16-bit number and prepends it with 169.254.

It performs a gratuitous ARP and assigns that address to itself.

The idea of Automatic Private IP Addressing is that two travelers could link their devices

quickly and easily. For instance, two train commuters could play a game on the way to the city.

They conﬁgure DHCP on their laptops and use Automatic Private IP Addressing on the train and

get a different IP in the ofﬁce.

If you see a 169.254.x.x address, it means that the DHCP server is not reachable. The PC will

not work because there is not a router to or from that IP. Troubleshoot this by ﬁnding out

why the PC cannot see the DHCP server.

DHCP Roles 387

Addresses are always assigned on the interface that has an IP address in the same subnet as the

pool. Example 13-1 shows this conﬁguration in context.

Some IOS devices receive an IP address on one interface and assign IP addresses on another. In

these instances, DHCP may import the options and parameters from one interface and pass

them to the other interface. The command to do so is

Router(config-dhcp)# ii

Using import all can reduce the headache of setting up DHCP in remote ofﬁces. The router, once

it is onsite, can determine the local DNS and options.

There are a variety of commands to manage an IOS DHCP server. The additional commands are

listed in Table 13-2.

Acting as a DHCP Relay

Normally, routers do not forward broadcast. There are a few times when an exception to this rule

would be useful. IP assignment, for instance, would be easier if you did not have to deploy a

DHCP server on every segment. If routers passed broadcasts, a central server could take care of

remote locations.

Example 13-1 DHCP

Router(config)# ii

Router(config-if)# ii

Router(config)# ii

Router(config-dhcp)# nn

Router(config-dhcp)# dd

Router(config-dhcp)# ll

Router(config-dhcp)# dd

Table 13-2 Descriptions of DHCP Server Commands

Command Description

service dhcp Enables DHCP server (on by default)

ip dhcp database Conﬁgures a database agent

no ip dhcp conﬂict logging Disables logging of conﬂicts

ip dhcp excluded-address start-ip end-ip Lists addresses that should be excluded from the pool

domain-name domain Default DNS assignment

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