Evaluation and Comparison of Spanning Tree Protocol and Rapid Spanning Tree Protocol on Cisco switches via OPNET

ENSC 427: COMMUNICATION NETWORKS

SPRING 2013

FINAL PROJECT

Project Group # 2

Joseph Lu 301077704 zla18@sfu.ca

Sen Jiang 301121645 senj@sfu.ca

Tao Xiong 301129494 txiong@sfu.ca

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Table of Contents

List of Figures ………………………………………………………………………………..….3

List of Tables …………………………………………………………………………………….4

Abstract .........................................................................................................................................5

1. Ethernet ……………………………………………………………………………………….5

1.1 Ethernet LAN ………………………………………………………………………………..5 1.2 Media Access Control (MAC) addressing ……………………………………………….….5 1.3 Switches ………………………….………………………………………………………….6 1.4 Virtual LANs ...........................................................................................................................6

2. Spanning Tree Protocol (STP) Overview ...............................................................................7

2.1 Spanning Tree Protocol (STP) ………………………………………………………………..7 2.2 Types of STPs ………………………………………………………………………………...9 2.2.1 Rapid Spanning Tree Protocol ……………………………………………………………9 2.2.2 Multiple Spanning Tree Protocol ………………………………………………………..11 2.3 Hypothesis …………………………………………………………………………………...11

3. OPNET Simulations ………………………………………………………………………...12

3.1 Topologies …………………………………………………………………………………..12 3.1.1 Three Layer Topology …………………………………………………………………..12 3.1.2 Ring Backbone Topology ……………………………………………………………….13 3.2 Simulation Setup …………………………………………………………………………….14 3.2.1 Simulation with Ring Backbone Topology ……………………………………………...14 3.2.2 Simulation with Three Layer Topology …………………………………………………19

4. Simulation Results …………………………………………………………………………..20

5. Conclusion and Discussion ………………………………………………………………….21

References ………………………………………………………………………………………23

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List of Figures

Figure.1 MAC Address ………………………………………………………………………….6

Figure.2 Spanning Tree Interface States [4] ...…………………………………………………...8

Figure.3 STP and Redundant Connectivity [4].………………………………………………….8

Figure.4 Root Port [5] …………………………………………………………………………..10

Figure.5 Designated Port [5] ……………………………………………………………………10

Figure.6 Alternate Port [5] ……………………………………………………………………...10

Figure.7 Backup Port [5] ………………………………………………………………………..10

Figure.8 Multiple Spanning Tree Protocol [7] ………………………………………………….11

Figure.9 Three Layer Topology [8] ……………………………………………………………..12

Figure.10 Ring Backbone Topology [8] ………………………………………………………...13

Figure.11 Scenario#1 and #2 ……………………………………………………………………14

Figure.12 Application Definition ………………………………………………………………..15

Figure.13 Profile Definition ……………………………………………………………………..15

Figure.14 Root Bridge Priority ………………………………………………………………….16

Figure.15 Workstation Attributes ……………………………………………………………….16

Figure.16 Server Attributes ……………………………………………………………………..17

Figure.17 Scenario#3 and Scenario#4 …………………………………………………………..17

Figure.18 Scenario#5 and Scenario#6 …………………………………………………………..18

Figure.19 Scenario#7 and Scenario#8 …………………………………………………………..18

Figure.20 Scenario#9 and Scenario#10 …………………………………………………………18

Figure.21 Scenario#11 and Scenario#12 ………………………………………………………..19

Figure.22 Scenario#13 and Scenario#14 ………………………………………………………..19

Figure.23 Results and Protocol Visualization of Scenario#1 and Scenario#2 ………………….20

Figure.24 Results and Protocol Visualization of Scenario#13 and Scenario#14 ……………….20

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List of Tables

Table.1 Port States of STP and RSTP [5] ………………………………………………………...9

Table.2 Project Specification ……………………………………………………………………14

Table.3 Results of All Scenarios ………………………………………………………………...21

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ABSTRACT

Spanning Tree Protocol (STP) was based on the algorithm invented by Radia Perlman to prevent loop forming in networks in 1985. [1] And in 1990, the IEEE published 802.1D as its first standard. It was introduced to any bridged Ethernet local area network. In 2001, the IEEE published Rapid Spanning Tree Protocol as 802.1w which provides significantly faster loop—free paths calculation in response to a topology change. Our project intends to evaluate and compare the performances of STP and RSTP on Cisco switches supported by OPNET. We were planning to do the comparison of STP with more other types such as Multiple Spanning Tree Protocol. However, after searching in OPNET V16.0, we found that in OPNET, Cisco switches only support STP and RSTP.

Our project first introduce some background information regarding VLAN, STP, RSTP, Layer Two Switches, Layer Three Switches, Distribution Layer, Access Layer, and so on. Besides the background information, the report discusses some companies that produce switches, especially the one's switches we will be using: Cisco. It then includes our work on OPNET. We build and configure the three layer topology and the ring backbone topology, then analyze and explain some snapshots taken from OPNET. We create STP tree with three layers which totally have approximately ten Cisco switches, and observe the performance of connection coming back up after disconnecting the loop. After finishing the STP evaluation, we run the same procedure for RSTP, and finally compare their results for a conclusion.

1. ETHERNET

1.1. Ethernet LAN

Ethernet is the most widely used local area network access model. It is standardized by the Institute of Electrical and Electronics Engineers (IEEE) as 802.3 [2] standard. An Ethernet environment involves various types of devices, such as hubs, bridges, and switches. A switching Ethernet environment is an Ethernet network that consists of switches instead of hubs. A switch is a more intelligent device than hub. It stores Media Access Control address in lookup table and maintain address at its own. There are switches that are used in different layers, such as layer two switches used in the data link layer which learn MAC address automatically, and are cheap and easy to deploy. Ethernet switches are able to link more than one local area network together.

1.2. Media Access Control (MAC) Addressing

Media Access Control address [2] is the physical address of the network device found in the data link layer aside from the logical address that is found in the network layer. A MAC address is a hardware identification serial number that uniquely identifies each device in the

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network. It is manufactured into every network adapter that differentiates network cards. Therefore, the MAC address cannot be modified. A MAC address is 48 bits (6 bytes) in length and usually written as a sequence of twelve hexadecimal digits. The first 24 bits (sixdigits) correspond to the organizational unique identifier (OUI) or the manufacturer’s unique identifier, while the last 24 bits (six digits) correspond to the device serial number that is assigned by the vendor, as illustrated in the figure below:

Figure.1 MAC Address

The combination of the OUI and the device serial number logically ensures that any of the network adapters have the different MAC address.

1.3. Switches

As bridges add Bridges add a level of intelligence to the network by using the MAC address to build a table of hosts, mapping these hosts to a network segment and containing traffic within these network segments. A switch functions the same as a bridge. When a switch receives a frame, it examines the destination and source MAC addresses and compares them to a table of network segments and addresses. The frame is dropped if the segments are the same, whereas forwarded to the proper segment if the segments are different.

In Ethernet LAN, switches are able to link several LANs together and forward frames between these LAN segments. They are used as a more intelligent version of hubs in networks.

1.4. Virtual LANs

A Virtual Local Area Network (VLAN) is a group of data exchanging ports of every switch that are chosen specifically for putting logic inside the switches correspondingly. Generally speaking, the function of VLANs is assigning tasks to switches through specific ports. VLANs can also be defined as broadcast domains due to the fact that broadcast packets are sent out all ports that are in the same VLAN. VLAN mapping configures layer two switches to provide the logical connectivity among the VLAN members. VLANs are standardized as IEEE 802.1Q.

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2. SPANNING TREE PROTOCOL (STP) OVERVIEW

2.1. Spanning Tree Protocol (STP)

The Spanning Tree Protocol (STP) is a link—management protocol under the Institute of Electrical and Electronics Engineers (IEEE) standard 802.1D for routing bridges and switches. The idea of Spanning Tree Protocol is using the spanning—tree algorithm to provide redundancy to your network without breaking it. When there is one link in the network fails, another way would automatically come up for the traffic to reach its destination. Due to that the network connection forms many loops which make data traffic fails finding the way to its destination, traffic congestion happens as data is transmitted around in circles. By using the STP algorithm, the switches that send bridge protocol data units (BPDUs) are able to identify active redundant links, and block one of these links to prevent any possible network loops. In a network, broadcast storms and constant table changes are created by network loops which bring down the entire network.

The spanning tree protocol creates a tree spanning across the entire network and forces redundant paths into a standby or blocking state in establishing path redundancy. In between two network devices such as switches, the spanning tree protocol allows only one active path at one time to prevent network loops. The redundant links that are established are used as a backup option in case that any link in primary path fails. Each spanning tree must have a root bridge that is effectively the root of the tree [3]. All active paths span out from this root. The root bridge is automatically selected by the spanning tree algorithm based on the lowest MAC address of bridges in the network with all bridge priorities are the same default values. Any users can modify the root bridge by setting one specific bridge configurable priority low.

Due to the propagation delays that occur when protocol information passes through a switched LAN, topology changes happen randomly in the switched network [4]. Every STP interface exists in one of the following states: Blocking, Listening, Learning, Forwarding, and Disabled. In blocking state, no participation takes place for the interface in frame forwarding. In listening state, the spanning tree protocol determines whether the interface should get involved in frame forwarding or not. In learning state, preparation takes place for the interface to participate in frame forwarding. In forwarding state, the interface forwards frames. Finally, in disabled state, a port is shut down with no active link or no spanning—tree instance running on it, which results in no participation for the interface in frame forwarding. The diagram below illustrates how an interface processes between each state:

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Figure.2 Spanning Tree Interface States [4]

An example of STP with redundant connectivity is provided by Cisco and illustrated in the graph below:

Figure.3 STP and Redundant Connectivity [4]

It is shown clearly that only one active path is available in the topology above with all other paths disabled.

Cisco [4] as well enhanced the original 802.1D standard with three different features to improve the spanning—tree convergence speed of a switched network: Port Fast, Backbone Fast, and Uplink fast. The port fast feature allows an interface to skip several spanning—tree states. The interface transits into forwarding state immediately after its blocking state. The backbone fast feature speeds up spanning—tree convergence by optimizing the max age which is 20 sec as the default setting. The uplink fast feature indicates that the backup link

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comes up and starts forwarding traffic as soon as the forwarding link fails. This feature is only configured on access switches.

2.2. Types of STPs

2.2.1. Rapid Spanning Tree Protocol (RSTP)

The rapid spanning tree protocol (RSTP) was developed based on the 802.1D spanning tree algorithm. It was first introduced by the Institute of Electrical and Electronics Engineers as 802.1w specification. The RSTP provides a significantly faster spanning—tree convergence in response to a topology change. As the STP convergence takes anywhere between 30 to 50 seconds depending on the type of failure to converge the network, the RSTP speeds up the converging process by integrating the first three states of the STP interface into one states: Discarding. As a result, the RSTP takes less convergence time for the port transit from blocking to forwarding during any topology change. For a purpose of clear illustration the differences of the port states between the STP and the RSTP, Cisco provides the table shown below:

Table.1 Port States of STP and RSTP [5]

Although the original 802.1D specification was enhanced by Cisco with the three features that are mentioned in the previous section, the enhanced version still has a drawback that the mechanisms are proprietary and need additional configuration [5].

In terms of configuring the RSTP, the following port roles now become variables that need to be determined: Root Port, Designated Port, Alternate Port, and Backup Port. Cisco also provides practical illustrations for better understanding of the port roles. [5]

The root port is the same as 802.1D which denotes the one port on each switch that has the best root path cost to the root. The exception that is worth mentioning is that the root bridge does not have any root ports.

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Figure.4 Root Port [5]

The designated port refers to the one port on a network segment that has the best

root path cost to the root.

Figure.5 Designated Port [5]

The alternate port represents a port that has an alternate path to the root. More

useful BPDUs are received by the alternate port from another bridgethat is a port blocked.

Figure.6 Alternate Port [5]

The backup port denotes a port that provides a redundant connection to a network

segment where another switch port connects.

Figure.7 Backup Port [5]

Any port roles can be in any of the three states indicated in Table.1.

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2.2.2. Multiple Spanning Tree Protocol (MSTP)

The Multiple Spanning Tree Protocol (MSTP) is considered as an evolution of the Spanning Tree Protocol and the Rapid Spanning Tree Protocol. It was first standardized in IEEE 802.1s and later merged into 802.1Q—2005. It was inspired from the Cisco proprietary Multiple Instances Spanning Tree Protocol (MISTP) [6]. The MSTP allows several VLANs to be mapped to a single spanning tree instance [6]. It forms MST regions which can be seen as a group of switches placed under a common administration sharing the same configuration attributes [6]. MST regions and all other bridges and LANs are connected into a single Common Spanning Tree (CST). The MSTP connects all devices and LANs with a single Common and Internal Spanning Tree (CIST) that supports automatic formation of each MST region, and it assigns frames to different VLANs to follow different paths that are separately based on Multiple Spanning Tree Instances (MSTIs). Cisco provides a simple diagram shown below that illustrates the functionalities and specifications of MSTP:

Figure.8 Multiple Spanning Tree Protocol [7]

Unfortunately, due to the fact that OPNET does not support MSTP for switches, it is not concerned in the project objectives.

2.3. Hypothesis STP (802.1D) and RSTP (802.1w) can both perform algorithms of creating loop

free paths in Ethernet networks.

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RSTP (802.1w) can response significantly faster to link failures than STP (802.1D) does.

3. OPNET SIMULATIONS

3.1. Topologies

The platform we choose for network implementations and simulations is the “Optimized Network Engineering Tools” (OPNET) that is widely used in research and in industries. By studying the IEEE 802.1D standard, “Media Access Control (MAC) Bridges”, we discovered two network topologies that can be used to implement our project: the Three Layer Topology inspired by the Cisco Hierarchical Network Model, and the Ring Backbone Topology.

3.1.1. Three Layer Topology

The graph below shows a typical Three Layer Topology:

Figure.9 Three Layer Topology [8]

The topology consists of three layers: the access layer (bottom), the distribution layer (middle), and the core layer (top).

The access layer is on the bottom that includes hubs and layer two switches. Layer two switches are switches used in the data link layer which are relatively cheaper and easier to deploy. The access layer mainly connects client nodes, such as workstations into Ethernet networks. Packets delivery to clients is secured by the functionality of the access layer.

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The distribution layer is in the middle that includes layer three switches and LAN—based routers. This layer connects servers into Ethernet networks. This layer confirms that packets are routed properly between subnets and VLANs. Layer three switches are switches used in the network layer which are more intelligent and expensive due to the fact that extra processing power and memory is required for operation comparing to lay two switches.

The core layer is on the top that includes high—end switches and high—speed cables. This layer ensures packets transmitting speed and reliability. However, there are no packet manipulations taking place in the core layer. Therefore, the core layer is not necessary to include in our project topology.

The three layer topology is inspired by the Cisco Hierarchical Network Model which optimizes the capacity, features, and functionality of a specific device for its position and role in the network. This fact promotes the network scalability and stability. [9]

3.1.2. Ring Backbone Topology

There is another topology that is widely used for protocol implementations in research and studies, the ring backbone topology which is also illustrated by the graph in the IEEE 802.1D standard:

Figure.10 Ring Backbone Topology [8]

The ring backbone topology is highly organized with all traffic flows being able to be transmitted in one direction at high speed. And for end user computers, each of them occupies equal access to resources. Furthermore, adding additional components would not affect the performance of the network. On the other hand, it also has some drawbacks such as the fact that packets must pass through all devices between the source and the destinations which makes it relatively slower than some other topologies.

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3.2. Simulation Setup

The configuration, implementation, and simulation in OPNET intend to evaluate and compare the performances in terms of link recovery responding speed of the Spanning Tree Protocol, and the Rapid Spanning Tree Protocol in the two topologies mentioned in previous section.

3.2.1. Simulation with Ring Backbone Topology

A network of the ring backbone topology that consists of five switches, one server, and one client workstation has been built and configured running with STP and RSTP separately. Firstly, a new project was created as the one contains all the scenarios. The settings of the Project Wizard dialog box are shown below in the table:

Initial Project Create Empty Project Click Next

Network Scale Campus Click Next

Specify Size 100 m X 100 m Click Next

Model Family Ethernet Click Next

Review Check Values Click Finish

Table.2 Project Specification

The first two scenarios include five switches (bridges) that run STP and RSTP correspondingly. An Application Definition Configuration node and a Profile Definition Configuration node are also added in the scenarios to provide a traffic application. The two scenario topologies are shown below:

Figure.11 Scenario#1 and #2

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In order to generate traffic, a video conferencing application has been chosen from the Application Definitions as it is shown below in the Figure 10:

Figure.12 Application Definition

As well, a Profile Definitions node is configured as follow:

Figure.13 Profile Definition

Besides the application definition and the profile definition, a Link Failure/Recovery node has also been added into the network. We set the link between Bridge_0 and Bridge_1 to fail at 160 seconds, and recover at 300 seconds.

Due to the fact that OPNET default setting of Root Bridge is the first switch that is put into the network, we manually edited the priority of Bridge_0 to 8192 to ensure it is the root bridge of the topology. The configuration dialog box is shown below:

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Figure.14 Root Bridge Priority

For the workstation node, we added the video conferencing application in its attributes and manually set its Client Address to “Des” as shown in Figure 13:

Figure.15 Workstation Attributes

We then added the video conferencing application in the attributes of the server node and assigned its application destination to address “Des” as shown in Figure 14:

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Figure.16 Server Attributes

For scenario#1, we edited the Bridge Parameters of all the switches under their attributes to STP, and as well for scenario#2 we edited them to RSTP.

We chose the relative statistic for collection and analysis. The statistic we chose is the point—to—point throughput in bit/sec between Bridge_1 and Workstation. The throughput in our project is the measurement of the video traffic passing over the specific link.

After finishing building and configuring the first two scenarios, we created ten more scenarios with different numbers of bridges. Scenario#3 and scenario#4 consist of three bridges, scenario#5 and scenario#6 consist of four bridges, scenario#7 and scenario#8 consist of six bridges, scenario#9 and scenario#10 consist of seven bridges, and scenario#11 and scenario#12 nine bridges running STP and RSTP correspondingly for each of the groups of the scenarios. The scenarios are show below:

Figure.17 Scenario#3 and Scenario#4

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Figure.18 Scenario#5 and Scenario#6

Figure.19 Scenario#7 and Scenario#8

Figure.20 Scenario#9 and Scenario#10

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Figure.21 Scenario#11 and Scenario#12

3.2.2. Simulation with Three Layer Topology

After creating all the scenarios for the Ring Backbone Topology, we then built two more scenarios: scenario#13 and scenario#14 presenting the three layer topology networks consist of three switches in the access layer and two switches in the distribution layer in order to evaluate and compare the performances of STP and RSTP in the three layer topology. The application definition node, the profile definition node, the server node and the workstation node are using the same setup as the Ring Backbone Topology scenarios. For the Link Failure/Recovery node, we edited the link between Bridge_0 (Root Bridge) and Bridge_4 to fail at 160 seconds and recover at 300 seconds. The statistic chosen is the point—to—point throughput in bit/sec between Bridge_4 and Workstation.

The network topology is shown in Figure 20:

Figure.22 Scenario#13 and Scenario#14

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For all the scenarios, the simulation run time is set to 500 seconds total.

4. Simulation Results

The aim is to collect and analyze the statistic that we choose. By using the function “Visualizing Spanning Tree”, OPNET automatically visualizes the traffic flow and failure link on the networks with blue and red emphasizing lines. Also, OPNET generates diagrams that display the point—to—point throughputs of each scenario. The result diagrams of scenario#1 and scenario#2 (Ring Backbone with five bridges), as well as the results diagrams of scenario#13 and scenario#14 (Three Layer with five bridges), are shown below in Figure 21 and Figure 22:

Figure.23 Results and Protocol Visualization of Scenario#1 and Scenario#2

Figure.24 Results and Protocol Visualization of Scenario#13 and Scenario#14

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As we can see clearly from the diagrams, for both topologies, STP (red line) and RSTP (blue line) both responded to the link failure at exactly 160 seconds. However, RSTP responded significantly faster than STP did to the link recovery. From the zoomed—in diagrams, the differences between STP and RSTP in recovery time are the same: RSTP responded at 299 seconds, and STP responded at 303 seconds. As well, the loop free paths and failed links are clearly presented in the network topologies.

We then simulated all the scenarios, and collected all the results inside the table below:

Scenarios Protocols Failure Time (sec) Recovery Time (sec)

Scenario#1 and 2 STP 160 303 RSTP 160 299

Scenario#3 and 4 STP 160 303 RSTP 160 299

Scenario#5 and 6 STP 160 303 RSTP 160 299

Scenario#7 and 8 STP 160 303 RSTP 160 299

Scenario#9 and 10 STP 160 303 RSTP 160 299

Scenario#11 and 12 STP 160 303 RSTP 160 299

Scenario#13 and 14 STP 160 303 RSTP 160 299

Table.3 Results of All Scenarios

5. Conclusion and Discussion

The project implementations and simulations can be considered successful even with some minor errors and technical difficulties occurred during the configuration. In short, the results that we obtained from simulation runs are satisfying.

By observing the spanning tree visualized networks, one can promptly conclude that both of the Spanning Tree Protocol (IEEE 802.1D) and the Rapid Spanning Tree Protocol (IEEE 802.1w) can generate loop free paths in networks for traffic to communicate between servers and workstations.

For both applicable topologies: the Three Layer Topology and the Ring Backbone Topology, the Rapid Spanning Tree Protocol requires much less calculation time in order to create the loop free paths in networks. Based on the results, the average time difference of recovery time between STP and RSTP is 3 seconds.

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Besides the fact that the results successfully evident the hypothesis we proposed in section 2, we also encountered some strange phenomenon from the result diagrams. We observed that for all the scenarios with different numbers of bridges, the same recovery time values were presented for STP and RSTP correspondingly. According to our research, the number of bridges in networks would definitely vary the calculation speed of spanning tree algorithm. We analyzed the case and concluded that the cause might be the topologies are simple enough for OPNET to minimize the delay for calculating loop free paths. Unfortunately, there have not been any chances for us to examine our supposition.

In the future, we would also like to evaluate the performance of the Multiple Spanning Tree Protocol in terms of calculating loop free paths using different regions of Spanning Tree Protocol, and compare it with the original STP (IEEE 802.1D) and RSTP (IEEE 802.1w). We did not implement MSTP because of OPNET lacking supportability of MSTP. Some other platforms such as NS—3 could be used for realization of MSTP implementation.

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References

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[2] Gerald W.B (Sep 25th, 2003), “The Second Information Revolution”, Harvard University Press. P. 151. ISBN 0—674—01178—3. Retrieved on Apr 17th, 2013.

[3] Perlman. R (1985). “Interconnections, Second Edition” USA: Addison—Wesley. ISBN: 0—201—63448—1. Retrieved on Apr 17th, 2013.

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[5] Cisco, “Understanding Rapid Spanning Tree Protocol (802.1w)”, Spanning Tree Protocol, Document ID: 24062, Oct 24th, 2006. Retrieved on Apr 17th, 2013. Internet: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfa.shtml

[6] Cisco, “Understanding Multiple Spanning Tree Protocol (802.1s)”, Spanning Tree Protocol, Document ID: 24248, Apr 17th, 2007. Retrieved on Apr 17th, 2013. Internet: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_white_paper09186a0080094cfc.shtml

[7] Cisco, “Understanding and Configuring Multiple Spanning Trees”, Catalyst 4500 Series Switch Cisco ISO software Configuration Guide, 12.2(25)EW. Retrieved on Apr 17th, 2013. Internet: http://www.cisco.com/en/US/docs/switches/lan/catalyst4500/12.2/25ew/configuration/guide/mst.html

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[10] Azcorra, A.; Ibanez, G. "Application of Rapid Spanning Tree Protocol for Automatic Hierarchical Address Assignment to Bridges". 25 Oct, 2004. Retrieved on Apr 17th, 2013. Internet: http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1341886

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[14] Cisco. “Troubleshooting LAN Switching Environments”. Document ID: 12006. 06 Oct, 2005. Retrieved on Apr 17th, 2013. Internet: http://www.cisco.com/en/US/tech/tk389/tk214/technologies_tech_note09186a0080176720.shtml#backinfo,