STUDY GUIDE
Basic Operations of Spanning Tree Protocol
Objectives
This module covers the following objectives in learning the concepts about the basic operations of
spanning tree protocol:
➢ Explain the differences and basic operations of spanning tree protocol.
➢ Perform and understand how to configure STP on a basic switched network.
➢ Familiarize yourself with the SHOW commands and other troubleshooting commands used in this
protocol.
Study Strategies
➢ Read and patiently understand the discussion presented in this module.
➢ Smartly rewrite in your notebook or make a summary of notes about the information being presented. By
this strategy, you will learn how to visualize the concepts and imagine how a switch fundamentally
operates in a network environment.
➢ Complete the challenge and the exercises at the end of the module. This will solidify the concepts that
you have learned through simulation. Simulation will be done by group and you need to collaborate with
your group members.
Introduction
We now have already the idea how switches behave and operate when connected to every device in the
network. These devices do not only include end-users or nodes but also network communicating devices such as
routers and other switches. However, when switches are connected to the other switches, especially, when forming
redundant links, a possible switching loop can occur. This switching loop can cause terrible problem on your
network.
So, for our discussion in this module, we will be tackling about how redundancies must need a good
mechanism to control switching loops and prevent possible major outage in our network.
fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
In the illustrated design, redundant links interconnect the switches. This idea is a good practice to ensure
continuous connectivity within the network, however, problem arises when a computer sends out a frame with a
broadcast, multicast, or unknown unicast destination MAC address. Recall that any of these transmissions causes
switch to flood that frame out to all its ports except for the one on which it was received.
JCQ.ECE 1
, ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
Say for example, PC A wants to talk to PC B. It then sends a broadcast, say an Address Resolution Protocol
(ARP) to find out where the location of PC_B. The green arrow shows a broadcast frame sent by PC_A. When
SW_A receives the broadcast frame, it forwards that frame to all ports except to the port where it was received.
SW_A forwards the ARP frame out of fa0/0 and fa0/1 ports. See figure above.
Suppose that SW_B receives the broadcast frame from fa0/0 first. It will then forward the frames out to
fa0/1 and fa0/2. See figure below.
ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
On the other hand, when the 2nd frame arrived on SW_B via port fa0/1, again, this frame will also be sent
out to fa0/0 and fa0/2. See figure below.
ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
As you can see, SW_A has sent two (2) broadcast frames out of its fa0/0 and fa0/1. SW_B then receives
each of them, creates two (2) copies and sends each one of them back to SW_A and to PC_B. When SW_A
receives these broadcast frames, it will continue re-broadcasting them again to its other interfaces and this will
keep on going on and on until you shutdown the network. This phenomenon is called a broadcast storm.
JCQ.ECE 2
Basic Operations of Spanning Tree Protocol
Objectives
This module covers the following objectives in learning the concepts about the basic operations of
spanning tree protocol:
➢ Explain the differences and basic operations of spanning tree protocol.
➢ Perform and understand how to configure STP on a basic switched network.
➢ Familiarize yourself with the SHOW commands and other troubleshooting commands used in this
protocol.
Study Strategies
➢ Read and patiently understand the discussion presented in this module.
➢ Smartly rewrite in your notebook or make a summary of notes about the information being presented. By
this strategy, you will learn how to visualize the concepts and imagine how a switch fundamentally
operates in a network environment.
➢ Complete the challenge and the exercises at the end of the module. This will solidify the concepts that
you have learned through simulation. Simulation will be done by group and you need to collaborate with
your group members.
Introduction
We now have already the idea how switches behave and operate when connected to every device in the
network. These devices do not only include end-users or nodes but also network communicating devices such as
routers and other switches. However, when switches are connected to the other switches, especially, when forming
redundant links, a possible switching loop can occur. This switching loop can cause terrible problem on your
network.
So, for our discussion in this module, we will be tackling about how redundancies must need a good
mechanism to control switching loops and prevent possible major outage in our network.
fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
In the illustrated design, redundant links interconnect the switches. This idea is a good practice to ensure
continuous connectivity within the network, however, problem arises when a computer sends out a frame with a
broadcast, multicast, or unknown unicast destination MAC address. Recall that any of these transmissions causes
switch to flood that frame out to all its ports except for the one on which it was received.
JCQ.ECE 1
, ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
Say for example, PC A wants to talk to PC B. It then sends a broadcast, say an Address Resolution Protocol
(ARP) to find out where the location of PC_B. The green arrow shows a broadcast frame sent by PC_A. When
SW_A receives the broadcast frame, it forwards that frame to all ports except to the port where it was received.
SW_A forwards the ARP frame out of fa0/0 and fa0/1 ports. See figure above.
Suppose that SW_B receives the broadcast frame from fa0/0 first. It will then forward the frames out to
fa0/1 and fa0/2. See figure below.
ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
On the other hand, when the 2nd frame arrived on SW_B via port fa0/1, again, this frame will also be sent
out to fa0/0 and fa0/2. See figure below.
ARP fa0/0 fa0/0
SW_A SW_B fa0/2
fa0/2
fa0/1 fa0/1
PC_A PC_B
As you can see, SW_A has sent two (2) broadcast frames out of its fa0/0 and fa0/1. SW_B then receives
each of them, creates two (2) copies and sends each one of them back to SW_A and to PC_B. When SW_A
receives these broadcast frames, it will continue re-broadcasting them again to its other interfaces and this will
keep on going on and on until you shutdown the network. This phenomenon is called a broadcast storm.
JCQ.ECE 2
