If you are familiar with switches and VLANs you might know that you require a router if you want to communicate between VLANs. In this lesson, I’ll show you how you can use a router connected to a single switch as a “router on a stick”.
This is the topology we’ll use:
On the switch we have VLAN 10 and VLAN 20 and there’s only a single cable between the router and switch. The router needs access to both VLANs so the link between the router and switch will be a trunk!
It’s possible to create sub-interfaces on a router. These are virtual interfaces and on each sub-interface we can configure an IP address, basically it looks like this:
You can pick any number that you like but I decided to use the VLAN numbers, one sub-interface for VLAN 10 and another for VLAN 20.
Here’s what the configuration looks like on the router:
R1(config)#interface fastEthernet 0/0
R1(config-if)#no shutdown
R1(config-if)#exit
R1(config)#interface fastEthernet 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 fastEthernet 0/0.20
R1(config-subif)#encapsulation dot1Q 20
R1(config-subif)#ip address 192.168.20.254 255.255.255.0Above you can see my two sub-interfaces and the IP addresses that I assigned to them. IP address 192.168.10.254 will be the default gateway for computers in VLAN 10 and 192.168.20.254 for computers in VLAN 20.
One important command is the encapsulation dot1Q. There is no way for our router to know which VLAN belongs to which sub-interface so we have to use this command. Fa0/0.10 will belong to VLAN 10 and Fa0/0.20 to VLAN 20. Let’s check the routing table:
R1#show ip route
Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2
i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level
ia - IS-IS inter area, * - candidate default, U - per-user static
o - ODR, P - periodic downloaded static route
Gateway of last resort is not set
C 192.168.10.0/24 is directly connected, FastEthernet0/0.10
C 192.168.20.0/24 is directly connected, FastEthernet0/0.20
You can see both sub-interfaces in the routing table. This allows the router to route between the two VLANs. That’s all there is to it! Configure your computers so that the router’s IP address for the corresponding VLAN is their default gateway and you are ready to go.
Configurations
Want to take a look for yourself? Here you will find the final configuration of each device.
R1
hostname R1
!
interface FastEthernet0/0
no ip address
duplex auto
speed auto
!
interface FastEthernet0/0.10
encapsulation dot1Q 10
ip address 192.168.10.254 255.255.255.0
!
interface FastEthernet0/0.20
encapsulation dot1Q 20
ip address 192.168.20.254 255.255.255.0
!
endSW1
hostname SW1
!
interface FastEthernet0/1
switchport trunk encapsulation dot1q
switchport mode trunk
!
interface FastEthernet0/2
switchport access vlan 10
switchport mode access
!
interface FastEthernet0/3
switchport access vlan 20
switchport mode access
!
endHOST1
hostname HOST1
!
no ip routing
!
interface FastEthernet0/0
ip address 192.168.10.1 255.255.255.0
duplex auto
speed auto
!
ip default-gateway 192.168.10.254
!
endHOST2
hostname HOST2
!
no ip routing
!
interface FastEthernet0/0
ip address 192.168.20.1 255.255.255.0
duplex auto
speed auto
!
ip default-gateway 192.168.20.254
!
endI hope you enjoyed this lesson! If you have any more questions just leave a comment below!
First, thank you for your helpful lesson. I have a question about vlan and this lesson. One reason for making vlan is security. If we do this (Router on a Stick) don’t you think we eliminate this security? Afterward every vlan can get to other vlan?
Thanks in Advance
Hi Regina,
The configuration on the Cisco switch is pretty straightforward. The interface connected to the router has to be a trunk:
And the interfaces that connect to the host are regular access ports:
interface fa0/1
You don’t need to use VLAN SVIs…a layer 2 switch uses the SVI only for management purposes. A layer 3 switch uses a SVI per VLAN which hosts can use as default gateway (in that case you don’t need a
... Continue reading in our forumHello Ian
If you don’t configure subinterfaces on the router, then no tagged traffic will enter the interface. The switch is set up to send frames to the router using dot1q encapsulation. This is where the VLAN number of each specific frame is added to the header as a tag. When the router receives these frames, it will drop them because they include a tag. By adding subinterfaces and the appropriate dot1q encapsulation, you are allowing the router to be able to receive tagged frames and to allow them to egress on the appropriate subinterface.
If communicati
... Continue reading in our forumHello Marit
By default a router interface does not have an IP address assigned. However, if you are working on a router that is not “out of the box” and has had previous configurations, then it’s always best practice to ensure that no IP address has been assigned to the physical interface. You could do this by examining the interface itself using
show ip interface brief, or you can simply issue theno ip addresscommand. It is considered a good precaution rather than a requirement to issue this command just to be sure there is no address assigned.I hope this has been helpful!
Laz
As usual very helpful, thanks Laz!