Configure DHCP Server for multiple VLANs on the Switch

This tutorial explains how to configure multiple DHCP pools on the DHCP server to support multiple VLANs. Learn how to configure DHCP on a switch to support multiple VLANs.

By default, switches forward broadcast messages. To control and limit broadcast messages, switches use VLANs. A VLAN defines a logical boundary for broadcast messages. If a VLAN is defined, the switch forwards broadcast messages only inside the VLAN.

Let's take a simple example to understand how VLAN controls broadcast messages.

A small network is built by using an 8 port switch. Until we configure a VLAN on it, it will forward an incoming broadcast message from all ports except the port on which the message came from.

Now, suppose we configure two VLANs on it and add the first four ports to the first VLAN and the last four ports to the second VLAN. After this configuration, when the switch receives a broadcast message on a port, it forwards the message only from the ports that belong to the same VLAN.

For example, if the switch receives a broadcast message on port 2, it will forward that message from ports 1, 3, and 4 only. Because, port 2 belongs to the first VLAN and other ports that belong to the first VLAN are 1, 3, and 4. Since ports 5, 6, 7, and 8 don't belong to the first VLAN, the switch will not forward the incoming broadcast message from these ports.

VLANs optimize networks by reducing the area of broadcast messages, but they cause problems for applications that rely on broadcast messages. For example, DHCP uses broadcast messages to enable communication between DHCP clients and the DHCP server.

If the DHCP server and clients are configured in different VLANs, clients will not receive IP configuration from the server. To solve this issue, we have two options. Either configure a DHCP server in each VLAN or configure multiple DHCP pools on the DHCP server and assign a DHCP pool to each VLAN.

We have already discussed the first option in the previous part of this article. In this part, we will learn how to create and configure multiple DHCP pools on the DHCP server and assign a DHCP pool to each VLAN through a packet tracer example.

Packet tracer lab setup

Either download the following pre-created packet tracer lab or create a packet tracer lab as described below.

Download link of the pre-created practice lab

Add a switch, a router, and six PCs to the workspace. Connect PCs to the switch and the switch to the router. The following image shows this lab.

packet tracer lab for practice dhcp server multiple vlans

Configure this lab to meet the following objectives.

  • Create three VLANs and add 2 PCs to each VLAN.
  • Configure the DHCP server on the switch and create three DHCP pools.
  • Assign a DHCP pool to each VLAN.
  • Configure all PCs as DHCP clients.
  • Verify that all PCs receive their IP configuration from the DHCP server.
  • Test connectivity between all PCs.

Creating and assigning VLANs

To create a VLAN, run the following command in global configuration mode.

Switch(config)#vlan [VLAN number or name]

To assign a VLAN to an interface, use the following command.

Switch(config-if)#switchport access vlan [VLAN number or name]

Create 3 VLANs: VLAN 10, VLAN 20, and VLAN 30 by running the following commands.

Switch>enable
Switch#configure terminal
Switch(config)#vlan 10
Switch(config-vlan)#exit
Switch(config)#vlan 20
Switch(config-vlan)#exit
Switch(config)#vlan 30
Switch(config-vlan)#exit
Switch(config)#

The following image shows how to execute the above commands on the packet tracer.

adding vlans

Assign VLAN 10 to Fa0/1 and Fa0/2, VLAN 20 to Fa0/3 and Fa0/4, and VLAN 30 to Fa0/5 and Fa0/6.

Switch(config)#interface FastEthernet 0/1
Switch(config-if)#switchport access vlan 10
Switch(config-if)#exit
Switch(config)#interface FastEthernet 0/2
Switch(config-if)#switchport access vlan 10
Switch(config-if)#exit
Switch(config)#interface FastEthernet 0/3
Switch(config-if)#switchport access vlan 20
Switch(config-if)#exit
Switch(config)#interface FastEthernet 0/4
Switch(config-if)#switchport access vlan 20
Switch(config-if)#exit
Switch(config)#interface FastEthernet 0/5
Switch(config-if)#switchport access vlan 30
Switch(config-if)#exit
Switch(config)#interface FastEthernet 0/6
Switch(config-if)#switchport access vlan 30
Switch(config-if)#exit
Switch(config)#

The following image shows the above configuration on the packet tracer.

assigning vlans to interfaces

Configuring trunk port

By default, all switch ports operate in access mode. In access mode, a switch port allows traffic of a single VLAN only. To allow traffic from multiple VLANs, we have to configure the port as the trunk port.

In our example, only the port that connects the switch and the router carries the traffic of multiple VLANs. To configure this port as a trunk port, use the 'switchport mode trunk' command in interface configuration mode.

Switch(config)#interface GigabitEthernet 0/1
Switch(config-if)#switchport mode trunk
Switch(config-if)#exit
Switch(config)#

The following image show this configuration on the packet tracer.

configure trunk port switch

Configuring DHCP server and creating DHCP pools

By default, DHCP service is enabled on all supporting Cisco switches. To use this service, we have to create and configure a DHCP pool on the switch.

The following table lists the steps that are required to create and configure a DHCP pool.

Configuration step Description (required command)
Define reserved IP addresses Use the 'ip dhcp excluded-address' command to define the range of reserved IP addresses.
Create the DHCP poolTo create a DHCP pool and enter DHCP pool configuration mode, use the 'ip dhcp pool [pool name]' command.
Define IP range for leaseuse the 'network [network ID] [subnet mask]' command to define the range of IP addresses. DHCP uses this range to provide IP configuration to clients.
Set default gateway IPTo set the default gateway IP address, use the 'default-router [IP address of the default gateway]' command.
Set DNS server IPTo set the DNS server IP address, use the 'dns-server [IP address of the DNS server]' command.

Create three DHCP pools by using the above configuration steps.

Switch(config)#ip dhcp excluded-address 10.0.0.1 10.0.0.10
Switch(config)#ip dhcp excluded-address 20.0.0.1 20.0.0.10
Switch(config)#ip dhcp excluded-address 30.0.0.1 30.0.0.10
Switch(config)#ip dhcp pool vPool10
Switch(dhcp-config)#network 10.0.0.0 255.0.0.0
Switch(dhcp-config)#default-router 10.0.0.1
Switch(dhcp-config)#dns-server 4.4.4.4
Switch(dhcp-config)#exit
Switch(config)#ip dhcp pool vPool20
Switch(dhcp-config)#network 20.0.0.0 255.0.0.0
Switch(dhcp-config)# default-router 20.0.0.1
Switch(dhcp-config)# dns-server 8.8.8.8
Switch(dhcp-config)#exit
Switch(config)#ip dhcp pool vPool30
Switch(dhcp-config)#network 30.0.0.0 255.0.0.0
Switch(dhcp-config)#default-router 30.0.0.1
Switch(dhcp-config)#dns-server 30.0.0.2
Switch(dhcp-config)#exit
Switch(config)#

The following image shows the above configuration on the packet tracer.

creating dhcp pools on switch

Assigning IP addresses to VLANs

DHCP uses the IP address of a VLAN to determine the pool from which it should provide an IP configuration. To assign an IP address to the VLAN, use the following commands.

Switch(config)#interface vlan [vlan name or number]
Switch(config-if)#ip address [ip address] [subnet mask]

Assign an IP address to each VLAN from the reserved IP address of the corresponding DHCP pool.

Switch(config)#interface vlan 20
Switch(config-if)#ip address 10.0.0.5 255.0.0.0
Switch(config-if)#exit
Switch(config)#interface vlan 20
Switch(config-if)#ip address 20.0.0.5 255.0.0.0
Switch(config-if)#exit
Switch(config)#interface vlan 30
Switch(config-if)#ip address 30.0.0.5 255.0.0.0
Switch(config-if)#exit
Switch(config)#

The following image shows the above configuration on the packet tracer.

assigning vlans to interfaces

Configuring the router's interface

By default, different VLANs cannot communicate with each other. To allow communication between different VLANs, we have to connect them through a router. We don't need a dedicated router interface for each VLAN. Routers support interface virtualization. We can use a single router interface to allow communication between multiple VLANs.

To virtualize the Fa0/0 interface and to configure it to allow communication between different VLANs, use the following configuration on the router.

Router>enable
Router#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface FastEthernet 0/0
Router(config-if)#no ip address
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#interface FastEthernet 0/0.10
Router(config-subif)#encapsulation dot1Q 10
Router(config-subif)#ip address 10.0.0.1 255.0.0.0
Router(config-subif)#exit
Router(config)#interface FastEthernet 0/0.20
Router(config-subif)#encapsulation dot1Q 20
Router(config-subif)#ip address 20.0.0.1 255.0.0.0
Router(config-subif)#exit
Router(config)#interface FastEthernet 0/0.30
Router(config-subif)#encapsulation dot1Q 30
Router(config-subif)#ip address 30.0.0.1 255.0.0.0
Router(config-subif)#exit
Router(config)#

The following image shows the above configuration on the packet tracer.

router on stick configuration

To learn how different VLANs communicate through the router in detail, you can check the following tutorial. It explains how to configure and use VLANs in detail.
VLAN Configuration Commands Step by Step Explained

Configuring and verifying DHCP clients

To configure a host as the DHCP client, click the host and click the Desktop menu option and click the IP configuration option and choose the DHCP option. If the DHCP server is properly configured, the DHCP client will receive the IP configuration in a few seconds.

The following image shows this procedure step by step.

verifying dhcp server configuration

If you prefer, you can use the ping command to test and verify connectivity between two hosts of different VLANs.

The following image shows how to use this command to test connectivity between PC1 and PC6. Since PC1 and PC6 belong to VLAN 10 and VLAN 30, respectively, a successful ping response verifies that both VLANs have proper connectivity.

dhcp sever supporting multiple vlans verify

Configured packet tracer lab

To download the configured lab of this example, use the following download link.

Configured packet tracer lab of the example

That's all for this part. In the next part of this tutorial, we will understand how to configure a DHCP server on a Cisco router. If you like this tutorial, please don't forget to share it with friends through your favorite social channel.

Advertisements

ComputerNetworkingNotes CCNA Study Guide Configure DHCP Server for multiple VLANs on the Switch