Configuring VLANs on a Cisco Catalyst Switch

VLANs let you segment a single physical switch into multiple isolated broadcast domains. Here’s the minimum viable configuration for two switches with a trunk between them.

1. Create the VLANs

Switch(config)# vlan 10
Switch(config-vlan)# name ENGINEERING
Switch(config-vlan)# exit

Switch(config)# vlan 20
Switch(config-vlan)# name SALES
Switch(config-vlan)# exit

2. Assign access ports

Switch(config)# interface range gi1/0/1 - 10
Switch(config-if-range)# switchport mode access
Switch(config-if-range)# switchport access vlan 10
Switch(config-if-range)# exit

Switch(config)# interface range gi1/0/11 - 20
Switch(config-if-range)# switchport mode access
Switch(config-if-range)# switchport access vlan 20
Switch(config-if-range)# exit

3. Configure the trunk

The uplink between switches needs to carry traffic for both VLANs:

Switch(config)# interface gi1/0/24
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
Switch(config-if)# exit

Repeat the trunk configuration on the matching port of the second switch.

4. Verify

Switch# show vlan brief
Switch# show interfaces trunk

show vlan brief confirms which ports belong to which VLAN. show interfaces trunk confirms the trunk is up and which VLANs are allowed across it.

Common gotcha

If two switches won’t pass traffic between VLANs after a trunk is configured, check that both ends agree on native VLAN and that switchport trunk allowed vlan lists match — a mismatch here silently drops traffic for the missing VLAN.