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Understanding Data Link Layer switches

There's a lot of Cisco material these days buzzing about multilayer switches and the benefits they can afford the internetwork.Network Management Software But if you're working in a production network or studying for your Cisco exams, what you really need is to be armed with a good grasp of exactly what a Layer 2 switch is and how a switch works within an internetwork.

In my last Daily Drill Down, you learned about the OSI model and how Cisco creates networks based on it. Cisco switches are primarily sold as Layer 2 (Data Link) devices. This should tell you that these switches perform their filtering and segmenting tasks using only the hardware (MAC) address of a given device.

By default, Layer 2 switches break up collision domains but not broadcast domains. You can purchase optional cards from Cisco that increase the switches functionality into the routing arena, thereby allowing them to also break up broadcast domains. But I'm not going there in this Daily Drill Down. First things, first. For now, we're only going to look into how Cisco switches are used in an internetwork at Layer 2.

As I said, Layer 2 switching is hardware-based, meaning it uses the Media Access Control (MAC) address from the host's network interface cards (NICs) to filter the network. Cisco switches use application-specific integrated circuits (ASICs) within their switches to build and maintain filter tables. Since all network segments aren't concerned with the same types of data, it's quite advantageous in terms of efficiency to be able to "shelter" various segments from having certain types of data unnecessarily traversing them. And switch filter tables do exactly that?athey exist to prevent unwanted or unneeded traffic from traversing certain network segments.

For a switch to provide this type of segmentation on the network, it creates something called a collision domain. Let's talk about collision domains in more detail.A collision domain is defined as a network segment that shares bandwidth with all other devices on the same network segment. When two hosts on the same network segment transmit at the same time, the resulting digital signals will fragment or collide, hence the term collision domain. It's important to know that a collision domain is found only in an Ethernet half-duplex network. Since most of today's LANs use some type of Ethernet network, collision domains are likely to be an important part of your network design.

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