LAN switching

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LAN switching is a form of packet switching used in local area networks. Switching technologies are crucial to network design, as they allow traffic to be sent only where it is needed in most cases, using fast, hardware-based methods.

Contents

Layer 2 switching

Layer 2 switching is hardware based, which means it uses the media access control address (MAC address) from the host's network interface cards (NICs) to decide where to forward frames. Switches use application-specific integrated circuits (ASICs) to build and maintain filter tables (also known as MAC address tables). One way to think of a layer 2 switch is as a multiport bridge.

Layer 2 switching provides the following

  • Hardware-based bridging (MAC)
  • Wire speed
  • High speed
  • Low latency
  • Low cost

Layer 2 switching is highly efficient because there is no modification to the data packet, only to the frame encapsulation of the packet, and only when the data packet is passing through dissimilar media (such as from Ethernet to FDDI). Layer 2 switching is used for workgroup connectivity and network segmentation (breaking up collision domains). This allows a flatter network design with more network segments than traditional 10BaseT shared networks. Layer 2 switching has helped develop new components in the network infrastructure

  • Server farms — Servers are no longer distributed to physical locations because virtual LANs can be created to create broadcast domains in a switched internetwork. This means that all servers can be placed in a central location, yet a certain server can still be part of a workgroup in a remote branch, for example.
  • Intranets — Allows organization-wide client/server communications based on a Web technology.

These new technologies allow more data to flow off from local subnets and onto a routed network, where a router's performance can become the bottleneck.

Limitations

Layer 2 switches have the same limitations as bridge networks. Bridges are good if a network is designed by the 80/20 rule: users spend 80 percent of their time on their local segment.

Bridged networks break up collision domains, but the network remains one large broadcast domain. Similarly, layer 2 switches (bridges) cannot break up broadcast domains, which can cause performance issues and limits the size of your network. Broadcast and multicasts, along with the slow convergence of spanning tree, can cause major problems as the network grows. Because of these problems, layer 2 switches cannot completely replace routers in the internetwork.

Layer 3 switching

The only difference between a layer 3 switch and router is the way the administrator creates the physical implementation. Also, traditional routers use microprocessors to make forwarding decisions, and the switch performs only hardware-based packet switching. However, some traditional routers can have other hardware functions as well in some of the higher-end models. Layer 3 switches can be placed anywhere in the network because they handle high-performance LAN traffic and can cost-effectively replace routers. Layer 3 switching is all hardware-based packet forwarding, and all packet forwarding is handled by hardware ASICs. Layer 3 switches really are no different functionally than a traditional router and perform the same functions, which are listed here

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