To help you understand more clearly about InterVLAN, the main configuration of router & switch are shown below:Ĭreate sub-interfaces, set 802.1Q trunking protocol and ip address on each sub-interface
Two popular trunking protocols in CCNA are 802.1q (open standard) and InterSwitch Link (ISL, a Cisco propriety protocol). + The router sub-interfaces must be running a trunking protocol. + The switch port connected to the router interface must be configured as trunk port. Let’s see what actions need to be completed when we want to configure InterVLAN in “router on a stick” model using the above topology. To accomplish InterVLAN routing, some configuration must be implemented on both router and switch. The router treats each sub-interface as a separate physical interface in routing decisions -> data can be sent and received in the same physical interface (but different sub-interfaces) without being dropped by the split-horizon rule in the case you want to send routing updates through the router from one VLAN to another. For example from a physical interface fa0/0 we can create many sub-interfaces like fa0/0.0, fa0/0.1 … Now this router is often called “router on a stick” (maybe because there is only one physical link connecting from router so it looks like a router on a stick ^^)
To overcome this problem, we can create many logical interfaces in one physical interface. Switch attaches VLAN information when receiving frames from host A and removes VLAN information before forwarding to host D.īut there is one disadvantage in the topology above: for each VLAN we need a physical connection from the router to the switch but in practical, the interfaces of the router are very limited. When frames leave the router (step 3 in the picture above), they are tagged with VLAN 20.Īlso notice that receiving ends (host A & D in this case) are unaware of any VLAN information. Notice that the routing decision to another VLAN is done by the router, not the switch. In turn, the router makes routing decision from VLAN 10 to VLAN 20 and sends back that traffic to the switch, where it is forwarded out to host D. The switch tags the frame as originating on VLAN 10 and forwards to the router. First, host A knows the destination host is in a different VLAN so it sends traffic to its default gateway (on the router) through the switch. Now let’s see how the traffic is sent from host A to host D. Now host A can communicate with host C or D easily. The routing traffic from one VLAN to another VLAN is called InterVLAN routing. To allow hosts in different VLANs communicate with each other, we need a Layer 3 device (like a router) for routing: But host A can’t communicate with host C or D because they are in different VLANs. For example, in the topology below host A and B can communicate with each other without a router in the same VLAN 10 host C and D can communicate in the same VLAN 20. But devices in separate VLANs require a Layer 3 routing device to communicate with one another. In this tutorial we will learn about InterVLAN Routing.Īs we learned, devices within a VLAN can communicate with each other without the need of Layer 3 routing.
In the previous VLAN tutorial we learned how to use VLAN to segment the network and create “logical” broadcast domains.