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Routing between Vlans using an external router

If you have ever heard the term ‘router on a stick’, that is a reference to using an external router connected using only one physical interface to a switch that has multiple vlans defined on it. The single physical interface is configured using vlan trunking/tagging with Ethernet subInterfaces.

Let’s assume there are three vlans configured for this small office scenario:

Vlan 2 - Cisco device management
Vlan 3 - Office Workstations
Vlan 4 - Servers

The vlan usage is pretty self explanatory with the descriptions above. The office PCs running Windows XP will be configured for Vlan 3. The office file/mail/print servers will be configured for Vlan 4. Lastly, Vlan 2 will be used as the management vlan for all managed network devices.

There will also be a vlan 999 for the native/untagged vlan traffic, since it is best practice not to use Vlan1 for this purpose.

Here is what the configurations would look like:

On the Cisco access switch:

hostname OfficeSwitch-001
vtp mode transparent
!
vlan 2
 name Management
vlan 3
 name Workstations
vlan 4
 name Servers
vlan 999
 name Native-Trunk
!
interface GigabitEthernet0/1
description Connected to OfficeRouter-001 Fa0/1
switchport trunk encapsulation dot1q
switchport trunk native vlan 999
switchport mode trunk
!
interface Vlan1
no ip address
!
interface Vlan2
ip address 10.1.2.10 255.255.255.0
!
ip default-gateway 10.1.2.1

On the 2811 router:

interface FastEthernet0/1
description Connected to OfficeSwitch-001 Gig0/1
no ip address
duplex auto
speed auto
!
interface FastEthernet0/1.2
description Management L3 Interface
encapsulation dot1Q 2
ip address 10.1.2.1 255.255.255.0
!
interface FastEthernet0/1.3
description Workstation L3 Interface
encapsulation dot1Q 3
ip address 10.1.3.1 255.255.255.0
!
interface FastEthernet0/1.4
description Server L3 Interface
encapsulation dot1Q 4
ip address 10.1.4.1 255.255.255.0
!
interface FastEthernet0/1.999
description Native Vlan
encapsulation dot1Q 999 native
no ip address

Author Info:

 
Scott is the Founder, Administrator, Architect, Chief Editor and Shameless Hack who wrote and runs Xpresslearn.
 

Scott has been working in the networking field for 13 years and has experience in all the areas: Lan, Wan, Security, Optimization, High Availability, Wireless, and others.

When he is not working on further development of Xpresslearn, Scott loves spending time with his family and has such hobbies as Camping, Fishing, Cars, and Woodworking.

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