Layer 2 switching

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Layer 2 switching

Today LANs are often implemented using Layer 2 switching, often referred to simply as switches. A switch effectively reduces the individual LANs to be the device and the switch. This approach means collisions no longer occur. Frames are inspected at the switch and forwarded to the appropriate destination port based on the Ethernet address. The switch learns which address is on which port by looking at the source address in the frames (if in doubt, the frame is forwarded on all ports). A side benefit of the switch architecture is the ability to mix port speeds. Access switches are often 10/100 MBps ports, distribution could be 100/1000 Mbps ports, and the switching core 1000/10000 Mbps.

Layer 2 switching in the enterprise network has become as important as routing. In the three-layer, hierarchical network identified by Cisco, Layer 2 switching is used in the core layer of the enterprise network. The reason is that Layer 2 switching provides very low latency, high reliability connections between distribution routers. With the addition of control protocols, these Layer 2 switches provide redundancy and fault tolerance at speeds difficult to obtain with a router.