Wednesday, February 15, 2012

Router on a stick

A router on a stick, is simply a router using 1 physical interface, but 2  logical interfaces to route between 2 vlans.

Lets say that VLAN2 and VLAN3 (the broadcast domains) exist on the switch, and the router's Fa0/0 is connected to switchport Fa0/1.

The switch trunks to the router on that port, sending the traffic for both VLANs, tagged via the trunking protocol.

On the router side, the IOS creates 2 sub interfaces, (fa0/0.100 and fa0/0.200 as an example) with 1 of them paying attention to frames tagged for vlan 2, and the other sub interface paying attention to the tags for vlan 3.   Each sub interface has one IP address, that matches the other devices that may live in that same vlan (2 or 3).  The devices in each point to the IP address of the router (in their vlan) as a default gateway.

Router example, for fa0/0

interface FastEthernet0/0.100
encapsulation dot1q 2
ip address
interface FastEthernet0/0.200
encapsulation dot1q 3
ip address
Switch connected to that router port example:
3560(config)#int FastEthernet 0/1
3560(config-if)#switchport trunk encapsulation dot1q
3560(config-if)#switchport mode trunk

The term, router on a stick, refers to the physical layout, of the Router, with a single connection to the switch.


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