Router-on-a-Stick: Route VLAN 10 and VLAN 20
LAB TOPOLOGY
Router-on-a-Stick Lab
One 802.1Q trunk · router subinterfaces as gateways
Scenario
Sales users are in VLAN 10 and Marketing users are in VLAN 20. SW1 is Layer 2 only, so R1 will provide the gateways through a single physical link using 802.1Q subinterfaces.
Addressing
| Device | Interface | Address / role |
|---|---|---|
| PC1 | NIC | 192.168.10.10/24, GW 192.168.10.254 |
| PC2 | NIC | 192.168.20.10/24, GW 192.168.20.254 |
| SW1 | E0/0 | trunk to R1 |
| R1 | E0/0.10 | 192.168.10.254/24, dot1q 10 |
| R1 | E0/0.20 | 192.168.20.254/24, dot1q 20 |
Configure SW1
SW1# configure terminal
SW1(config)# vlan 10
SW1(config-vlan)# name SALES
SW1(config-vlan)# vlan 20
SW1(config-vlan)# name MRKT
SW1(config-vlan)# exit
SW1(config)# interface ethernet 0/1
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 10
SW1(config-if)# exit
SW1(config)# interface ethernet 0/2
SW1(config-if)# switchport mode access
SW1(config-if)# switchport access vlan 20
SW1(config-if)# exit
SW1(config)# interface ethernet 0/0
SW1(config-if)# switchport trunk encapsulation dot1q
SW1(config-if)# switchport mode trunk
SW1(config-if)# end
Configure R1
R1# configure terminal
R1(config)# interface ethernet 0/0
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface ethernet 0/0.10
R1(config-subif)# encapsulation dot1q 10
R1(config-subif)# ip address 192.168.10.254 255.255.255.0
R1(config-subif)# exit
R1(config)# interface ethernet 0/0.20
R1(config-subif)# encapsulation dot1q 20
R1(config-subif)# ip address 192.168.20.254 255.255.255.0
R1(config-subif)# end
Verify before pinging
SW1# show interfaces trunk
SW1# show vlan brief
R1# show ip interface brief
R1# show running-config interface ethernet 0/0.10
R1# show running-config interface ethernet 0/0.20
Now ping PC2 from PC1. If it fails, verify PC gateway first, then VLAN membership, trunk state, subinterface encapsulation and R1 interface state—in that order.
Break it
Change R1 E0/0.20 encapsulation to VLAN 30 while SW1 still sends VLAN 20. Predict which VLAN fails and prove it with trunk/subinterface output before fixing it.
Reference
VLAN IDs, trunk interface concept, subinterface pattern and gateway addresses follow the Router-on-a-Stick section of the supplied CCNA All LABS PDF.