STP Setup. Compex Technologies WPE54, WPE54G, Network Card WPE54

Chapter 5 Further Configuration

Spanning Tree Protocol (STP) is a link management protocol that helps to prevent undesirable loops occur in the network. For an Ethernet network to function properly, only one active path can exist between two stations. If a loop exists in the network topology, duplication of messages will occur and this might confuse the forwarding algorithm and allow duplicate frames to be forwarded.

In short, the main purpose of activating STP is to prevent looping when you have redundant paths in the network. Without activating STP, redundant topology will cause broadcast storming.

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Chapter 5 Further Configuration

To establish path redundancy, STP creates a tree that spans all of the switches in an extended network, forcing redundant paths into a standby, or blocked, state. but establishes the redundant links as a backup if the initial link should fail. If STP costs change, or if one network segment in the STP becomes unreachable, the spanning tree algorithm reconfigures the spanning tree topology and re-establishes the link by activating the standby path. Without spanning tree in place, it is possible that both connections may be simultaneously live, which could result in an endless loop of traffic on the LAN.

Spanning-Tree Protocol operation is transparent to end stations, which are unaware whether they are connected to a single LAN segment or a switched LAN of multiple segments.

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Chapter 5 Further Configuration

The figure shown below explains the implementation of STP in a network. AP#1 is physically connected to a switch whilst another 4 access points (AP#2, AP#3, AP#4 and

AP#5) are connected to AP#1 wirelessly. Redundant paths were found in this network, without enabling STP function, broadcast storm will occur in this network, resulted duplicated frames to be forwarded.

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Chapter 5 Further Configuration

When STP is enabled, the STP-enabled access points will first try to find the root access point using the following criteria: a. use the access point that is configured with the smallest STP priority.

Default priority set in the access points is 32768. b. If the STP priority values are the same, the access point with smallest MAC address will be chosen as root.

Once the root access point is determined, the STP algorithm will start to calculate the path cost from each access point to the root access point. Based on the path cost in the following table,

4Mbps 250

10Mbps 100

16Mbps 62

45Mbps 39

100Mbps 19

155Mbps 14

622Mbps 6

1Gbps 4

10Gbps 2

The path with the smallest cost will be used and extra redundant paths will be disabled.

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To explain the effect of STP & Pseudo VLAN on the wireless clients, we will compare 3 separate scenarios.

Scenario #1 – (No STP, No Pseudo VLAN)

Referring to the illustration below, if the Spanning Tree Protocol (STP) and Pseudo

VLAN are not implemented in a network, all clients (Notebook#1, #2, #3 & #4,) can access to one another, resulting low level of data security. If redundant paths were found in this network, broadcast packets will be duplicated and forwarded endlessly resulting in a broadcast storm.

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Scenario #2 – (With STP, No Pseudo VLAN)

When STP is enabled, extra redundant network paths between access points will be disabled, hence preventing multiple active network paths in between any two network access points.

If one of the access points is down, the STP algorithm will reactivate one of the redundant paths so that the network connection will not be lost.

All wireless users will be able to communicate with each other if they are associated to the access points of the same WDS zone.

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Scenario #3 – (With STP and Pseudo VLAN)

In this example, both STP and Pseudo VLAN are implemented in this network. All wireless users are unable to communicate with one another. This is one of the measures to ensure data privacy between wireless users in the network.

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Chapter 5 Further Configuration

Enabling STP Setup

Click on STP Setup from the CONFIGURATION menu

Select Enable from the STP State radio button.

STP State:

Activate Spanning Tree Protocol

(STP) function makes your network more resilient to link failure and also provides a protection from loop.

Priority:

Specify the configurable value that is appended as the most significant portion of a AP.

This value specifies which access point acts as the central reference point, or Root AP, for the STP system — the lower the priority value, the more likely the access point is to become the Root AP. If the priority values are all the same, then the system will search for the smallest MAC address of the access point and set it as the Root AP.

Hello Time:

Specify the time in seconds that elapses between the configuration messages (also known as

Hello BPDUs) generated by an AP that assumes itself to be the Root AP.

Forwarding Delay:

Specify the time in seconds that an AP spends in the listening and learning states, that is, listening for configuration messages.

Max Aging Time:

Specify the maximum age in seconds at which the stored configuration message information is judged to be too old and is discarded.

If an AP does not receive a configuration message after the Max Aging Time, the system will assume that the link between itself and the Root AP has gone down and will then reconfigures the network to cater for the change.

Click on the Apply button.

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