The way the layers are
implemented depends on the needs of the network being designed. Note, however, that for a network to function optimally, hierarchy must be maintained.
With respect to the hierarchical model, traditional campus LANs have followed one of two
designs—single router and distributed backbone—as shown in Figure 12–13.
Designing Switched LAN Internetworks 12-25
Switched LAN Network Designs
Figure 12-13
Traditional campus design.
Core
FDDI
dual ring
Distribution
Router
Router
Router
Access
Single-router design
Distributed backbone design
In the single-router design, the core and distribution layers are present in a single entity—the router.
Core functionality is represented by the backplane of the router and distribution is represented by the router. Access for end users is through individual- or chassis-based hubs. This design suffers from scalability constraints because the router can be only be in one physical location, so all segments end at the same location—the router. The single router is responsible for all distribution functionality, which can cause CPU overload.
The distributed backbone design uses a high-speed backbone media, typically FDDI, to spread
routing functionality among several routers. This also allows the backbone to traverse floors, a
building, or a campus.
Switched LAN Network Design Principles
When designing switched LAN campus networks, the following factors must be considered:
• Broadcast radiation—Broadcast radiation can become fatal—that is, 100 percent of host CPU
cycles can be consumed by processing broadcast and multicast packets. Because of delays
inherent in carrier sense multiple access collision detect (CMSA/CD) technologies, such as
Ethernet, any more than a small amount of broadcast traffic will adversely affect the operation of devices attached to a switch. Although VLANs reduce the effect of broadcast radiation on all
LANs, there is still a scaling issue as to how many hosts should reside on a given VLAN. A router allows for larger network designs because a VLAN can be subsegmented depending on traffic
patterns. However, in a nonoptimal network design, a single router can be burdened with large
amounts of traffic.
• Well-behaved VLANs—A well-behaved VLAN is a VLAN in which 80 percent or more of the
traffic is local to that VLAN. In an example in which the Marketing, MIS, and Engineering
departments each have an individual VLAN segment, the 80 percent rule is violated when a user
in the Marketing VLAN reads mail from the MIS VLAN, mounts servers from the Engineering
VLAN, and sends e-mail to members of the Engineering VLAN.
• Available bandwidth to access routing functionality—Inter-VLAN traffic must be routed, so the network design must allocate enough bandwidth to move inter-VLAN traffic from the source,
through the device that provides routing functionality, and to the destination.
• Appropriate placement of administrative boundaries—Switching has the effect of flattening networks, and the deployment of switching outside of your administrative boundary can
adversely affect the network within your administrative boundary.
12-26
Cisco CCIE Fundamentals: Network Design
Switched LAN Network Design Principles
Campus network designs are evolving rapidly with the deployment of switching at all levels of the network—from the desktop to the backbone. Three topologies have emerged as generic network
designs:
• Scaled Switching
• Large Switching/Minimal Routing
• Distributed Routing/Switching
Scaled Switching
The scaled switching design shown in Figure 12–14 deploys switching at all levels of the network
without the use of routers. In this design, each layer consists of switches, with switches in the access layer providing 10-Mbps Ethernet or 16-Mbps Token Ring to end users.
Figure 12-14
Scaled switching design.
Core
Switches
100 Mbps
Distribution
Switches
100 Mbps
Access
Switches
10-Mbps Ethernet
or 16-Mbps Token Ring
Scaled switching is a low-cost and easy-to-install solution for a small campus network. It does not require knowledge of address structure, is easy to manage, and allows all users to communicate with one another. However, this network comprises a single broadcast domain. If a scaled switched
network needs to grow beyond the broadcast domain, it can use VLANs to create multiple broadcast
domains. Note that when VLANs are used, end users in one VLAN cannot communicate with end
users in another VLAN unless routers are deployed.
Large Switched/Minimal Routing
The large switched/minimal routing design deploys switching at the access layer of the network,
either ATM switching or LAN switching at the distribution layer of the network, and ATM/LAN
switching at the core. Figure 12–15 shows an example of this network design.
Designing Switched LAN Internetworks 12-27
Switched LAN Network Designs
Figure 12-15
Large switched/minimal routing design.
Core
Switches
Switches
155 Mbps
100 Mbps
Distribution
Router
Router
Router
155 Mbps
100 Mbps
Switches
Switches
155 Mbps
100 Mbps
Access
Switches
10-Mbps Ethernet
or 16-Mbps Token Ring
In the case of ATM in the distribution layer, the following key issues are relevant:

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	instantiation of each layer can be in distinct routers or switches, can be represented by a physical media, can be combined in a single device, or can be omitted altogether...	Pomodliłem się do każdego boga jaki istniał bym był w wstanie wkurzyć tę kobietę do granic możliwości.