Cisco Systems 1800 User Manual

CHAPTER

5

Configuring a LAN with DHCP and VLANs

The Cisco 1800 series integrated services fixed-configuration routers support clients on both physical
LANs and virtual LANs (VLANs). The routers can use the Dynamic Host Configuration Protocol
(DHCP) to enable automatic assignment of IP configurations for nodes on these networks.

Figure 5-1 shows a typical deployment scenario with two physical LANs connected by the router and

two VLANs.

Figure 5-1 Physical and Virtual LANs with DHCP Configured on the Cisco Router

3

1

2

4

92339

1 Fast Ethernet LAN (with multiple networked devices)
2 Router and DHCP server—Cisco 1800 series integrated services router—connected to the Internet
3 VLAN 1
4 VLAN 2

DHCP

DHCP, which is described in RFC 2131, uses a client/server model for address allocation. As an
administrator,you can configure your Cisco 1800 integrated services fixed-configuration router to act as
a DHCP server, providing IP address assignment and other TCP/IP-oriented configuration information
to your workstations. DHCP frees you from having to manually assign an IP address to each client.

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Cisco 1800 Series Integrated Services Routers (Fixed) Software Configuration Guide

5-1

Configure DHCP

Note Whenever you change server properties, you must reload the server with the configuration data from the

Chapter 5 Configuring a LAN with DHCP and VLANs

When you configure a DHCP server, you must configure the server properties, policies, and DHCP
options.

Network Registrar database.

VLANs

The Cisco 1800 series integrated services routers (fixed) support eight Fast Ethernet ports on which you
can configure VLANs.

VLANs enable networks to be segmented and formed into logical groups of users, regardless of the
user’s physical location or LAN connection.

Configuration Tasks

Perform the following tasks to configure this network scenario:

• Configure DHCP

• Configure VLANs

Note The procedures in this chapter assume you have already configured basic router features as well as

PPPoE or PPPoA with NAT. If you have not performed these configurations tasks, see Chapter 1, “Basic

Router Configuration,” Chapter 3, “Configuring PPP over Ethernet with NAT,” and Chapter 4,
“Configuring PPP over ATM with NAT” as appropriate for your router.

Configure DHCP

Perform these steps to configure your router for DHCP operation, beginning in global configuration
mode:

Command Purpose

Step 1

Step 2

ip domain name name

Example:

Router(config)# ip domain name smallbiz.com
Router(config)#

ip name-server server-address1

[server-address2...server-address6]

Example:

Router(config)# ip name-server 192.168.11.12
Router(config)#

Identifiesthedefaultdomainthattherouterusesto
complete unqualified hostnames (names without a
dotted-decimal domain name).

Specifies the address of one or more Domain Name
System (DNS) servers to use for name and address
resolution.

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Cisco 1800 Series Integrated Services Routers (Fixed) Software Configuration Guide

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Chapter 5 Configuring a LAN with DHCP and VLANs

Command Purpose

Step 3

ip dhcp excluded-address low-address

[high-address]

Example:

Router(config)# ip dhcp excluded-address

192.168.9.0

Configure DHCP

Specifies IP addresses that the DHCP server should
not assign to DHCP clients. In this example, we are
excluding the router address.

Step 4

Step 5

Step 6

Step 7

ip dhcp pool name

Example:

Router(config)# ip dhcp pool dpool1
Router(config-dhcp)#

network network-number [mask | prefix-length]

Example:

Router(config-dhcp)# network 10.10.0.0

255.255.255.0

Router(config-dhcp)#

import all

Example:

Router(config-dhcp)# import all
Router(config-dhcp)#

default-router address [address2...address8]

Example:

Router(config-dhcp)# default-router 10.1.1.1
Router(config-dhcp)#

Creates a DHCP address pool on the router and
enters DHCP pool configuration mode. The name
argument can be a string or an integer.

Definessubnet number (IP) address for the DHCP
address pool, optionally including the mask.

Imports DHCP option parameters into the DHCP
portion of the router database.

Specifiesupto 8 default routers for a DHCP client.

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Step 8

Step 9

Step 10

dns-server address [address2...address8]

Example:

Router(config-dhcp)# dns-server 192.168.35.2
Router(config-dhcp)#

domain-name domain

Example:

Router(config-dhcp)# domain-name cisco.com
Router(config-dhcp)#

exit

Example:

Router(config-dhcp)# exit
Router(config)#

Cisco 1800 Series Integrated Services Routers (Fixed) Software Configuration Guide

Specifies up to 8 DNS servers available to a DHCP
client.

Specifies the domain name for a DHCP client.

Exits DHCP configuration mode, and enters global
configuration mode.

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