The graphic highlights the changes that will take place as the
nonhierarchical network is redeployed in the new Enterprise Composite Network
model strategy.
There are four objectives in the design of any high
performance network: security, availability, scalability, and manageability.
The Enterprise Composite Network model, when implemented properly, provides the
framework to meet these objectives. In the migration from a current network
infrastructure to the Enterprise Composite Network model, a number of
infrastructure changes may need to occur including the replacement of current
equipment and existing cable plant.
This list describes the equipment
and cabling considerations that should be made when altering infrastructure:
- Replace hubs and legacy switches with new switches at the Building Access
layer. Select equipment with the appropriate port density at the Access layer
to support the current user base, and plan for growth. Some designers plan for
about 30 percent growth up front. If the budget allows, use modular access
switches to accommodate future growth. Consider planning for support of inline
power and QoS if IP telephony may be implemented in the future.
- When building the cable plant from the Access layer to the Building
Distribution layer devices, remember that these links will carry aggregate
traffic from the end nodes at the Access layer to the Building Distribution
switches. Ensure that these links have adequate bandwidth capability.
EtherChannel bundles can be used here to add bandwidth as necessary.
- At the building distribution layer, select switches with adequate
performance to handle the load of the current Building Access layer. Also plan
some port density to add trunks later to support additional access layer
devices. The devices at this layer should be multilayer switches that support
routing between the workgroup VLANs and network resources. Depending on the
size of the network, the building distribution layer devices selected may be
fixed chassis or modular. Plan for redundancy in the chassis, and in the
connections to the access and core layers as the business objectives
dictate.
- The campus backbone equipment must support high-speed data communications
between other submodules. Be sure to size the backbone for scalability and plan
in redundancy.
Cisco has online tools for assisting designers in making the proper
selection of devices and uplink ports based on business and technology needs.
Cisco suggests oversubscription ratios that can be used to plan bandwidth
requirements between key devices on a network with average traffic flows.
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Access to distribution layer links – The oversubscription ratio can
be 20:1. That is, the link can be 1/20 of the total bandwidth available
cumulatively to all end devices using that access to distribution layer
link.
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Distribution to Core links – The ratio should be 4:1.
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Between Core Devices – There should be little to no oversubscription
planning. That is, the links between core devices should be able to carry
traffic at the speed represented by the aggregate number bandwidth of all the
Distribution uplinks into the core.
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CAUTION:
These ratios are appropriate for estimating average traffic from
access layer, end user devices. They are not accurate for planning
oversubscription from the Server Farm or Edge distribution modules. They are
also not accurate for planning bandwidth needed on access switches hosting
atypical user applications with high bandwidth consumption. (e.g. non client
-server databases or multimedia flows to unicast addresses. Using QoS end to
end prioritizes the traffic which would need to be dropped in the event of
congestion.
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