In this series, I would like to post my practical experience in setting up multiple VLANs/subnets and using a Layer 3 switch to route traffic across them. The post will also include my configuration steps enabling the use of a Microsoft-based DHCP Server. This is by no means a treatise on these subjects. A number of sites exist with more detailed and in-depth information of the subjects.

VLANs basically divide broadcast domains. Usually, a subnet is mapped to a specific VLAN. Devices across subnets/VLANs need a router or a Layer 3 switch to communicate. I used the following devices in this lab:

a) Cisco 3750G Enhanced Multilayer Image
b) Cisco 2960 10/100 Layer 2 Switch
c) MS DHCP Server

1) We will start by configuring the Cisco 3750G Switch. Configure the vlans we intend to route:

Switch> enable
Password:
Switch# config t (To enter the global configuration mode)
Enter configuration commands, one per line. End with CNTL/Z.
Switch(config)# hostname L3Switch
L3Switch(config)#vlan 20 (create new vlan)
L3Switch(config-vlan)#exit
L3Switch(config)# vlan 30
L3Switch(config-vlan)#exit
L3Switch(config)#vlan 40
L3Switch(config-vlan)#exit
L3Switch(config)# do show vlan (Use the show vlan command to display and confirm the existing vlans)

2) Enable IP routing on the Cisco 3750G

L3Switch# config t
L3Switch(config)# ip routing ( Enables IP Routing on the Cisco 3750 Layer 3 Switch)

In the next post, I will show how to create and configure the Switch Virtual Interfaces and configure the helper address on the 3750G. I will also post my configuration for the 2960 Access switches.