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2-2 Multilayer IP/IPX Switching in the LAN
MARTIN TAYLOR
When the first LANs were installed in enterprise networks in the early 1980s, 10 Megabits per secondeven when shared among several hundred usersseemed an extraordinary amount of bandwidth. And compared with the 2400 and 9600 baud communications links commonly used to connect dumb terminals to mini and mainframe computers, 10 Mbps was indeed generous!
The inventors of those original LANs promised that the advent of open communications networking between distributed computing resources would fundamentally change the landscape of information processing systems, even if they weren't, at the time, very clear about the exact nature of this change. It took quite a few years for the full implications of open networking to emerge: but now the LAN is ubiquitous, and the new vocabulary of network computing is expanding at a breathtaking pace. Who could have foreseen three years ago that the graphical web browser would lead to such an explosive growth of Internet usage? Or that this same technology would be taken up with such enthusiasm in corporate and enterprise networksnow increasingly referred to as intranets?
As LANs move beyond the era of simple mainframe connectivity and file and print services to encompass client/server computing, workflow applications, intranet Web server access, and real-time voice and video communications, so the technology of LANs has needed to undergo a continual overhaul. More and more powerful computing systems drive the network even harder, and larger and more information-rich data objects create a continually growing network traffic load. Traffic patterns are changing too, as workgroup- or department-level server-based solutions give way to corporate servers, pushing unprecedented levels of traffic across the LAN backbone.
To keep up with this growth in network activity, LAN technologies have evolved in two dimensionsspeed and segmentation. Speed simply delivers more bandwidth: Ethernet has added 100 Mbps and (soon) 1 Gbps variants, Token Ring went from 4 to 16 Mbps, while FDDI and ATM have extended the speed choice still further. Segmentation enables more of this bandwidth to be delivered to individual users or hosts. Segmentation technologies have evolved from local bridging through collapsed backbone routing to LAN switching; and since switching enables dedicated bandwidth to be delivered economically to each user, this is clearly where the future of high performance LANs lies.
But today, LAN switching does not provide a complete and general solution for large-scale LAN installations. This is because conventional LAN switching is not completely scalable. In most practical networks today, LAN switches must be used in conjunction with routers. We will come to the reasons for this later in this chapter. But in practice, having to combine switches with routers to build LAN infrastructures has a major impact in a number of areas:
- From the viewpoint of the network's users, it divides LAN communications into two classes of performance. Packets traveling on direct switched connections enjoy the high speed and consistent journey times of the expressway, but packets that are forced to pass through routers take the slow road, and suffer further delays when traffic is heavy.
- Switches and routers are separate pieces of equipment that must be separately purchased, configured and managed, and inevitably the costs of implementing a solution based on these multiple elements are greater than for a solution based on a single, integrated approach.
- The use of routers in the LAN results in additional administration to deal with moves and changes. For example, if a PC is moved from a LAN segment on one router port to a different LAN segment connected to another router port, it is usually necessary to allocate a new IP address to the PC and reconfigure this address in the PC's software.
Multilayer switching is based on an intelligent combination of switching and routing technologies to provide a complete and integrated solution for all kinds of LAN infrastructures, which very effectively addresses each of these issues. That's why more and more network planners will soon be setting their sights on a migration to multilayer switching in the LAN. (See Exhibit 2-2-1.)
Exhibit 2-2-1. Convergence of Switching and Routing
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