Inter-vlan routing

There are three inter-VLAN routing options:

• Legacy Inter-VLAN routing - This is a legacy solution. It does not scale well.

• Router-on-a-Stick - This is an acceptable solution for a small to medium-sized network.

• Layer 3 switch using switched virtual interfaces (SVIs) - This is the most scalable solution for medium to large organizations.

Legacy inter-VLAN routing using physical interfaces works, but it has a significant limitation. It is not reasonably scalable because routers have a limited number of physical interfaces. Requiring one physical router interface per VLAN quickly exhausts the physical interface capacity of a router.

Note: This method of inter-VLAN routing is no longer implemented in switched networks and is included for explanation purposes only.

Router-on-a-Stick Inter-VLAN Routing

The ‘router-on-a-stick’ inter-VLAN routing method overcomes the limitation of the legacy inter-VLAN routing method. It only requires one physical Ethernet interface to route traffic between multiple VLANs on a network.

A Cisco IOS router Ethernet interface is configured as an 802.1Q trunk and connected to a trunk port on a Layer 2 switch. Specifically, the router interface is configured using subinterfaces to identify routable VLANs.

The configured subinterfaces are software-based virtual interfaces. Each is associated with a single physical Ethernet interface. Subinterfaces are configured in software on a router. Each subinterface is independently configured with an IP address and VLAN assignment. Subinterfaces are configured for different subnets that correspond to their VLAN assignment. This facilitates logical routing.

When VLAN-tagged traffic enters the router interface, it is forwarded to the VLAN subinterface. After a routing decision is made based on the destination IP network address, the router determines the exit interface for the traffic. If the exit interface is configured as an 802.Q subinterface, the data frames are VLAN-tagged with the new VLAN and sent back out the physical interface.

Inter-VLAN Routing on a Layer 3 Switch

The modern method of performing inter-VLAN routing is to use Layer 3 switches and switched virtual interfaces (SVI). An SVI is a virtual interface that is configured on a Layer 3 switch, as shown in the figure.

Note: A Layer 3 switch is also called a multilayer switch as it operates at Layer 2 and Layer 3. However, in this course we use the term Layer 3 switch.

Inter-VLAN SVIs are created the same way that the management VLAN interface is configured. The SVI is created for a VLAN that exists on the switch. Although virtual, the SVI performs the same functions for the VLAN as a router interface would. Specifically, it provides Layer 3 processing for packets that are sent to or from all switch ports associated with that VLAN.

The following are advantages of using Layer 3 switches for inter-VLAN routing:

• They are much faster than router-on-a-stick because everything is hardware switched and routed.

• There is no need for external links from the switch to the router for routing.

• They are not limited to one link because Layer 2 EtherChannels can be used as trunk links between the switches to increase bandwidth.

• Latency is much lower because data does not need to leave the switch in order to be routed to a different network.

• They more commonly deployed in a campus LAN than routers.

The only disadvantage is that Layer 3 switches are more expensive.