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Local VLANs
Finance Sales Sales Finance Support Sales Support
Si
Si Si
Core
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Chapter 11 
Design Considerations
Redundancy is one of the key aspects of this design model. The Spanning Tree Protocol (STP)
is used to ensure a loop-free layer 2 network. This allows multiple links between an Access layer
switch and the Distribution layer, where STP will block all but one connection between them.
It can take up to 30 seconds for a redundant connection to start forwarding packets, and so
because of this delay, enhancements have been made to the STP algorithm. These enhancements
include Cisco’s uplink-fast and 802.1w, or Rapid Spanning Tree Protocol (RSTP), and can
achieve sub-second failover to a redundant link. Sometimes the Access layer utilizes a layer 3
switch and provides for smaller broadcast domains, which takes some of the pressure from the
Distribution layer