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4.2 | |
Introduction to LAN Switching | |
| 4.2.7 | |
Layer 2 and layer 3 switching |
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Switching is the process of receiving an
incoming frame on one interface and delivering that frame out another
interface. Routers use Layer 3 switching to route a packet. Switches use
Layer 2 switching to forward frames. The difference between Layer 2 and Layer 3 switching is the type of information inside the frame that is used to determine the correct output interface. Layer 2 switching is based on MAC address information. Layer 3 switching is based on network layer addresses or IP addresses. Layer 2 switching looks at a destination
MAC address in the frame header and forwards the frame to the appropriate interface
or port based on the MAC address in the switching table.
Layer 3 switching is a function of the
network layer. The Layer 3 header information is examined and the packet
is forwarded based on the IP address.
Traffic flow in a switched or flat network is inherently different from the traffic flow in a routed or hierarchical network. Hierarchical networks offer more flexible traffic flow than flat networks.
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The
switching table is contained in Content Addressable Memory (CAM). If the Layer 2 switch does not know where to send the frame,
it broadcasts the frame out all ports to the network. When a reply is
returned, the switch records the new address in the CAM. 