EN - 'Router on a Stick' or Inter VLAN routing

Hey again,

I try to explain in simple steps how to do inter vlan routing.
Due to the lack of real equipment @home I used dynamips from the GNS3 package.

I built the folowing setup using 3725 routers.
R0 is the real router.
R2 is used as switch. In Slot1 I inserted a NM-16ESW which simulates the switch.
Host1 and Host2 are used as host only.



I will only post non-auto config lines.

R0

hostname R0
!
interface FastEthernet0/0
 no ip address
 speed 100
 full-duplex
!
interface FastEthernet0/0.100
 encapsulation dot1Q 100
 ip address 10.0.100.1 255.255.255.0
!
interface FastEthernet0/0.200
 encapsulation dot1Q 200
 ip address 10.0.200.1 255.255.255.0
!
interface FastEthernet0/1
 no ip address
 shutdown
 duplex auto
 speed auto
!
ip route 10.0.100.0 255.255.255.0 FastEthernet0/0.100
ip route 10.0.200.0 255.255.255.0 FastEthernet0/0.200

For each VLAN a subinterface has to be created. I recommend using the VLAN number but there are no rule for it.
When configuring the subinterfaces enter first the encapsulation command.
encapsulation dot1Q <vlan ID>

To enable routing between VLANs you need either static routes or a routing protocol. I used static routes pointing for each subnet to its related subinterface.

R2

hostname R2
!
interface FastEthernet1/0
 description Link to R0
 switchport mode trunk
 duplex full
 speed 100
!
interface FastEthernet1/5
 description Link to Host1
 switchport access vlan 100
 duplex full
 speed 100
!
interface FastEthernet1/10
 description Link to Host2
 switchport access vlan 200
 duplex full
 speed 100

Due to this router is used as Switch only, there are no IP addresses configured. In a real scenario there would be at least a management IP normaly bound on a VLAN.

Host1

hostname Host1
!
interface FastEthernet0/0
 ip address 10.0.100.100 255.255.255.0
 speed 100
 full-duplex
!
ip route 0.0.0.0 0.0.0.0 FastEthernet0/0

Due to these routers are (mis)used as host, the have an default gateway pointing at the outgoing interface.

Host2

hostname Host2
!
interface FastEthernet0/0
 ip address 10.0.200.200 255.255.255.0
 speed 100
 full-duplex
!
ip route 0.0.0.0 0.0.0.0 FastEthernet0/0

I highly recommend (in every environment) to use the speed and duplex settings to avoid running in mismatch errors!

For questions just use the comment section.
Regards,
Zif