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26 Chapter 1: Design Overview
Other Design Models
Depending on the business requirements, all three tiers might not be required. Small and
medium businesses might be able to use models that implement only one or two tiers of the
hierarchical model.
NOTE
When using the one- or two-tier model, be sure to incorporate scalability and adaptability for
network expansion. As the business grows, the three-tier design might be required in the future.
One-Tier
In a one-tier design, the router operates at the core layer, providing connectivity between the
users and the WAN. Routers connected to the core in the one-tier model are typically the same
routers that might be used at the access layer of the hierarchical model. For a low-speed WAN
backbone, slow performance can be a factor. Use available features on the Cisco router to
improve WAN performance and utilization:
·
Header, link, and payload compression
·
Priority queuing and bandwidth reservation
·
Proxy services between the router and clients
·
Encapsulation and tunneling across the WAN core
Figure 1-12 shows examples of the one- and two-tier models.
Figure 1-12
One- and Two-Tier Models
Two-Tier
In a two-tier design, core routers provide access to the WAN, and access routers give client
LANs access to corporate resources. Core routers use features that take advantage of expensive
WAN resources.
Low-speed WAN
backbone
Clients and
servers
Use for small,
geographically
dispersed
internetworks
87200333.book Page 26 Wednesday, August 22, 2001 2:18 PM