Specifications

Introduction April 2014
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Larger LAN designs require a dedicated distribution layer for network-based services versus sharing connectivity
with access layer devices. As the density of WAN routers, WAAS controllers, Internet edge devices, and wireless
LAN controllers grows, the ability to connect to a single distribution layer switch becomes hard to manage. There
are a number of factors that drive LAN design with multiple distribution layer modules:
• The number of ports and port bandwidth that the distribution layer platform can provide affects network
performance and throughput.
• Network resilience is a factor when all LAN and network-based services rely on a single platform,
regardless of that platform’s design, it can present a single point of failure or an unacceptably large
failure domain.
• Change control and frequency affects resilience. When all LAN, WAN, and other network services are
consolidated on a single distribution layer, operational or configuration errors can affect all network
operation.
• Geographic dispersion of the LAN access switches across many buildings in a larger campus facility
would require more fiber optic interconnects back to a single collapsed core.
Figure 5 - Network-services distribution layer
WAN
2087
LAN
Distribution
Layer
Client
Access
Switches
LAN
Core
Internet
Network Services
Distribution Layer
Wireless
LAN Controller
Firewall
Like the access layer, the distribution layer also provides QoS for application flows to guarantee critical
applications and multimedia applications perform as designed.
Core Layer
In a large LAN environment there often arises a need to have multiple distribution layer switches. One reason
for this is that when access layer switches are located in multiple geographically dispersed buildings, you can
save potential costly fiber-optic runs between buildings by locating a distribution layer switch in each of those
buildings. As networks grow beyond three distribution layers in a single location, organizations should use a core
layer to optimize the design.
Another reason to use multiple distribution layer switches is when the number of access layer switches
connecting to a single distribution layer exceeds the performance goals of the network designer. In a modular
and scalable design, you can colocate distribution layers for data center, WAN connectivity, or Internet edge
services.