8
Chapter 1
Scaling Large Internetworks
segment, each one enjoys instant access to the full lot of available bandwidth
instead of competing for it with other users. Because of this, the collisions
that are common with shared, medium-sized networks that use hubs (half-
duplex) just don't happen.
A LAN switch bases the forwarding of frames on the frame's Layer 2
address (Layer 2 LAN switch) or on the Layer 3 address of the packet (multi-
layer LAN switch). LAN switches are sometimes referred to as
LAN frame
switches
because they generally forward Layer 2 frames in contrast to an
ATM switch, which forwards cells. Do not confuse this with Frame Relay,
which is a WAN technology.
LAN switches uniquely support some very cool new features, including
the following:
Numerous, simultaneous conversations
High-speed data exchanges
Low latency and high frame-forwarding rates
Dedicated communication between devices
Full-duplex communication
Media rate adaptation (10,100, and 1000Mbps hosts can work on the
same network)
The ability to work with existing 802.3-compliant network interface
cards and cabling
Thanks to dedicated, collision-free communication between network
devices, file-transfer throughput is increased. Many conversations can occur
simultaneously by forwarding or switching several packets at the same time,
which expands the network capacity by the amount of supported conversations.
The Cisco Three-Layer Model
C
isco has created its own three-layer hierarchical model. The Cisco
hierarchical model is used to help you design, implement, and maintain a
scalable, reliable, cost-effective hierarchical internetwork. Cisco defines
three layers of hierarchy, as shown in Figure 1.3, each with specific functionality.
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