Cisco switch: 802.1q trunk to router, aka router-on-a-stick

Connect a Cisco switch and router via 802.1q trunking. This configuration is known as a router-on-a-stick.

First, let’s start with the switch. VLAN 1 exists by default, so we’ll add a couple of other VLANs:

vlan 100
name data
state active

vlan 200
name data
state active

Now, let’s configure the switchport to connect to the router:
interface fastethernet 1/0/1
switchport trunk encapsulation dot1q
switchport mode trunk

Over to the router!

interface fastethernet 0/0
no ip address

interface fastethernet 0/0.1
encapsulation dot1q 1 native

interface fastethernet 0/0.100
encapsulation dot1q 100
ip address 192.168.100.1 255.255.255.0

interface fastethernet 0/0.200
encapsulation dot1q 200
ip address 192.168.200.1 255.255.255.0

At this point, workstations on VLAN 100 should be able to ping the 192.168.100.1 address, and workstations on VLAN 200 should be able to ping the 192.168.200.1 address.

If, for some reason, the native VLAN on the switchport is something other than 1, change the encapsulation statement on the router, or the trunk may not form.

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courtesy  http://www.tech-recipes.com/rx/1853/Cisco_switch_802_1q_trunk_to_router_on_a_stick/