Traditional switching operates at layer 2 of the OSI model, where packets are sent to a specific switch port based on destination MAC addresses. Routing operates at layer 3, where packets are sent to a specific next-hop IP address, based on destination IP address. Devices in the same layer 2 segment do not need routing to reach local peers. What is needed however is the destination MAC address which can be resolved through the Address Resolution Protocol (ARP) as illustrated below:
Since VLANs exist in their own layer 3 subnet, routing will need to occur for traffic to flow in between VLANs. This is where a layer 3 switch can be utilized. A Layer 3 switch is basically a switch that can perform routing functions in addition to switching. A client computer requires a default gateway for layer 3 connectivity to remote subnets. When the computer sends traffic to another subnet, the destination MAC address in the packet will be that of the default gateway, which will then accept the packet at layer 2, and proceed to route the traffic to the appropriate destination based on its routing table.
switch layers
The diagram below shows an example of a layer 3 switching routing between VLANs through its two VLAN interfaces. As before, the layer 3 device will still need to resolve the MAC address of PC B through an ARP request broadcasted out to VLAN 20. It then rewrites the appropriate destination MAC address and forwards the packet back out the layer 2 segment:
The Cisco hierarchical model, also known as the hierarchical internetworking model, is a three-layer model for network design which is widely used in industry. It divides enterprise networks into three layers: core, distribution and access layer. As the lowest level of Cisco three-tier network model, the access layer is responsible for providing end user devices with a connection to network resources. Access layer devices contain a hub, multi-station access unit and switch. In this article, we will put emphasis on the access switch. In the meanwhile, some important features of the access switch will be introduced as well.
