Information Technology Reference
In-Depth Information
identify failure points. It is more difficult to troubleshoot if hierarchical design is not used
because the network is not divided into segments.
To d a y 's fa s t - c o n v e r g i n g p r o t o c o l s w e r e d e s i g n e d fo r h i e r a r c h i c a l t o p o l o g i e s . To c o n t r o l
the impact of routing-protocol processing and bandwidth consumption, you must use
modular hierarchical topologies with protocols designed with these controls in mind, such
as the Open Shortest Path First (OSPF) routing protocol.
Hierarchical network design facilitates route summarization. Enhanced Interior Gateway
Routing Protocol (EIGRP) and all other routing protocols benefit greatly from route sum-
marization. Route summarization reduces routing-protocol overhead on links in the net-
work and reduces routing-protocol processing within the routers. It is less possible to
provide route summarization if the network is not hierarchical.
Hierarchical Network Design
As shown in Figure 2-1, a traditional hierarchical LAN design has three layers:
The core layer provides fast transport between distribution switches within the enter-
prise campus.
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The distribution layer provides policy-based connectivity.
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The access layer provides workgroup and user access to the network.
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To Enterprise
Edge Modules
Core
Distribution
Access
Figure 2-1
Hierarchical Network Design Has Three Layers: Core, Distribution, and Access
Each layer provides necessary functionality to the enterprise campus network. You do not
need to implement the layers as distinct physical entities. You can implement each layer in
one or more devices or as cooperating interface components sharing a common chassis.
Smaller networks can “collapse” multiple layers to a single device with only an implied hi-
erarchy. Maintaining an explicit awareness of hierarchy is useful as the network grows.
Core Layer
The core layer is the network's high-speed switching backbone that is crucial to corporate
ing characteristics:
