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OSI Reference Model

Introduction to TCP/IP

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Understanding Switching

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What Is Layer 3 Switching?

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Understanding LAN Switching

Broadcasts Consume Bandwidth

Now, in terms of broadcast, it's relatively easy to broadcast in a network, and that's a transmission mechanism that many different protocols use to communicate certain information, such as address resolution, for example.Address resolution is something that all protocols need to do in order to map Layer 2 MAC addresses up to logical layer, or Layer 3, addresses. For example, in an IP network we do something known as an ARP, an Address Resolution Protocol.And this allows us to map Layer 3 IP addresses down to Layer 2 MAC-layer addresses. Also, in terms of distributing routing protocol information, we do this by way of broadcasting, and also some key network services in our networks rely on broadcast mechanisms as well.

And it doesn't really matter what our protocol is, whether it's AppleTalk or Novell IPX, or TCP IP, for example, all of these different Layer 3 protocols rely on the broadcast mechanism. So, in other words, all of these protocols produce broadcast traffic in a network.

Broadcasts Consume Processor Performance

Now, in addition to consuming bandwidth on the network, another by-product of broadcast traffic in the network is that they consume CPU cycles as well.Since broadcast traffic is sent out and received by all stations on the network, that means that we must interrupt the CPU of all stations connected to the network.So here in this diagram you see the results of a study that was performed with several different CPUs on a network. And it shows you the relative level of CPU degradation as the number of broadcasts on a network increases.

So you can see, we did this study based on a SPARC2 CPU, a SPARC5 CPU and also a Pentium CPU. And as the number of broadcasts increased, the amount of CPU cycles consumed, simply by processing and listening to that broadcast traffic, increased dramatically.So, the other thing we need to recognize is that a lot of times the broadcast traffic in our network is not needed by the stations that receive it.So what we have then in shared LAN technologies is our broadcast traffic running throughout the network, needlessly consuming bandwidth, and needlessly consuming CPU cycles.

Hub-Based LANs

So hubs are introduced into the network as a better way to scale our thinand thick Ethernet networks. It's important to remember, though, that these are still shared Ethernet networks, even though we're using hubs.

Basically what we have is an individual desktop connection for each individual workstation or server in the network, and this allows us to centralize all of our cabling back to a wiring closet for example. There are still security issues here, though.It's still relatively easy to tap in and monitor a network by way of a hub. In fact it's even easier to do that because all of the resources are generally located centrally.If we need to scale this type of network we're going to rely on routers to scale this network beyond the workgroup, for example.

It's makes adds, moves and changes easier because we can simply go to the wiring closet and move cables around, but we'll see later on with LAN switching that it's even easier with LAN switching.Also, in terms of our workgroups, in a hub or concentrator based network, the workgroups are determined simply by the physical hub that we plug into. And once again we'll see later on with LAN switching how we can improve this as well.

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