3-7 Implementing Routing, Switching, and VLANs in Modern Corporate Networks
THOMAS ATKINS
Corporate networks have undergone dramatic evolutionary changes over the past several years. New computing paradigms, more powerful desktop and server platforms, and the ever increasing capabilities of network equipment have altered the way network managers architect networks.
The first corporate networks consisted of large (Ethernet or Token Ring) segments connected with repeaters. The only way to extend the size of the network was to add repeaters. These simple devices regenerated LAN signalingallowing more users to share a network that could span a larger geographical area. Repeaters were simple devices that propagated packets (both good and error types) across the segment.
Repeater environments gave way to bridged networks. Bridges allowed network managers to divide repeated environments into two or more segments. Network users on both sides of the bridge could transparently share data, but error frames were isolated to the segment on which they originated. Bridges increased network performance by allowing the creation of multiple segments, but scalability issues plagued large networks. Installations with many bridges were difficult to troubleshoot, and broadcast storms in one location could affect users throughout the enterprise.
Eventually, routers were introduced to corporate networks. These devices solved many of the problems inherent to bridged networks. Routers control broadcasts, particularly across low bandwidth wide area network (WAN) connections, and are deployed widely in most corporate networks. However, while providing much needed scalability characteristics, routers imposed a performance penalty for processing each packet. This performance hit (typically about 20 milliseconds) was not initially noticed on large networks (due to the other benefits provided by routers) increased traffic loads and the adoption of intranet computing eventually highlighted router performance limitations.
Switches arrived in corporate networks in the early 1990s and quickly earned a reputation as an excellent performance enhancement to router networks. Switches performed the same functions as bridges, but were designed to forward traffic at much higher rates. Switches increased performance within the local area network (LAN) environment, but rarely provided the WAN and broadcast control abilities of traditional routers. Many modern networks now contain both routers and switches each performing separate functions to provide a combination of performance and management functions.
The latest type of device to be introduced to corporate networks are switches that can also perform router functions. This new generation of equipment (Layer 3 switch) is capable of forwarding traffic either as a switch or a router without imposing the performance penalty typically associated with routing. Layer 3 switches promise to change the way modern networks are designed for the remainder of the decade.
NETWORK DESIGN EVOLUTION
As network devices have evolved, network designs have changed to make the best use of the new capabilities afforded by the latest equipment. But even though several new types of devices have been introduced over the years, network designs have focused on three key points: performance, management, and redundancy. New devices have addressed one or more of these points, allowing network designers to further enhance portions of their networks with each new piece of equipment.
Focus on Performance
Improving performance was the first enhancement network managers could make to their developing networks years ago. Repeated environments became slower as more users were added to the same shared media. Increased traffic and contention made it more difficult for each user to access the network. When bridges arrived, performance could easily be improved. Each two-port bridge effectively doubled network bandwidth by segmenting a single repeated segment into two collision domains. Performance was increased because available network bandwidth was shared among fewer users. Broadcast traffic, however, could still propagate across the bridged domain.
As with bridges, switches allowed network managers to make further improvements in network performance. Switches were deployed in much the same way as bridges, but were introduced long after router technology had matured.
As mentioned earlier, bridges and switches allow network managers to increase performance by segmenting collision domains into smaller segments containing fewer users. Unfortunately, these devices also propagate broadcasts, which can create problems when large numbers of bridges and/or switches are deployed in a single broadcast domain. This scalability issue, inherent to bridged/switched network designs is typically addressed by the focus on management.
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