This document describes a case study on network design involving configuring spanning tree protocol on an enterprise network. The network was initially configured with redundant links but traffic was not flowing as expected. The task is to configure spanning tree with Core_2 as the root bridge and adjust link parameters so that primary links are used over redundant links. Checking the traffic patterns and spanning tree output after making the changes is also part of the task.

1. CCNA Discovery: Designing and Supporting Computer Networks
Chapter 2 Case Study
Network Design
Note: This case study utilizes Packet Tracer. Please see the Chapter 2 Packet Tracer file located in
Supplemental Materials.
Introduction and Scenario
The enterprise network is now performing much better than it did when you were first requested to
improve it. You have arranged for extensive redundant links to be added to the network, and you feel
comfortable that internally it is now less prone to a single point of failure. However, you notice that
when you monitor traffic on the network, some of the links that you designed to be used only as “back‐
ups” appear to be carrying traffic, and links you assumed would be primary links are inactive. Suddenly,
you realize what is happening!
Now that you have multiple potential loops, the spanning tree algorithm is doing exactly what it should
do, which is making certain that there are no loops in the network. The problem is that the spanning
tree process is completely random, since you haven’t made any changes to the network that will
determine how it should operate. For instance, which links should be “redundant” and which switch
should become the root bridge. You must do something about this.
© 2009 Cisco Learning Institute

2. CCNA Discovery: Designing and Supporting Computer Networks
Chapter 2 Case Study
Having the potential to disrupt traffic when the spanning tree protocol recomputes as you make your
planned changes, you send an “all stations” email advising that the network will be disrupted for a
couple of hours that evening, and resign yourself to a long night ahead.
Task 1
You have determined that you want the Core #2 switch to be the root switch in the spanning tree. Make
the necessary changes to ensure that Core #2 is always chosen as the root. (Hint: don’t use zero!). You
also want Core #1 to be the designated backup root bridge.
Scenario 2
Now that the spanning tree is recomputed with Core_2 as the root, you can look at traffic patterns again
and see how the links are behaving. However, it is still not quite as you would like. You determine that
additional changes are required.
Task 2
Adjust the necessary parameters on the distribution switches and access switches so that the primary
links are used rather than the redundant links (Hint: a faster link has a lower cost!)
Note: It is considered that Core #2 is the primary core switch for Distribution switches 3 and 4 and
Core #1 for Distribution switches 1 and 2.
Access 1 is primary for Distribution 1, Access 2 is primary for Distribution 2… etc (the “crossover”
links are considered the redundant links).
In addition, the links between Distribution 1 and 2 and between 3 and 4 are considered
redundant links.
Make the priority of the distribution switches one increment above Core #1 ‐ which should be one
increment above Core #2
Reflection
Check the traffic patterns and compare with the given answer. Use the show command to check the
results on the spanning tree on various switches and compare the output. Can you analyze each line?
Does it agree with the clear representation you can see? Why does the priority value appear to be one
greater than what you set?
© 2009 Cisco Learning Institute