Allied Telesis AT-GS950/8 manual Spanning Tree Overview, Bridge Priority Root

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Chapter 8: Spanning Tree Protocol (STP)

Spanning Tree Overview

Bridge Priority

and the Root

Bridge

The performance of a Ethernet network can be negatively impacted by the formation of a data loop in the network topology. A data loop exists when two or more nodes on a network can transmit data to each other over more than one data path. The problem that data loops pose is that data packets can become caught in repeating cycles, referred to as broadcast storms, that needlessly consume network bandwidth and can significantly reduce network performance.

The Spanning Tree Protocol (STP) prevents data loops from forming by ensuring that only one path exists between the end nodes in your network. Where multiple paths exist, these protocols place the extra paths in a standby or blocking mode, leaving only one main active path.

STP can also activate a redundant path if the main path goes down. So not only do these protocols guard against multiple links between segments and the risk of broadcast storms, but they can also maintain network connectivity by activating a backup redundant path in case a main link fails.

Where the two protocols differ is in the time each takes to complete the process referred to as convergence. When a change is made to the network topology, such as the addition of a new bridge, a spanning tree protocol must determine whether there are redundant paths that must be blocked to prevent data loops, or activated to maintain communications between the various network segments. This is the process of convergence.

With STP, convergence can take up to a minute to complete in a large network. This can result in the loss of communication between various parts of the network during the convergence process, and the subsequent lost of data packets.

The STP implementation on the AT-S82 management software complies with the IEEE 802.1d standard.

The first task that bridges perform when a spanning tree protocol is activated on a network is the selection of a root bridge. A root bridge distributes network topology information to the other network bridges and is used by the other bridges to determine if there are redundant paths in the network.

A root bridge is selected by the bridge priority number, also referred to as the bridge identifier, and sometimes the bridge’s MAC address. The bridge with the lowest bridge priority number in the network is selected as the root bridge. If two or more bridges have the same bridge priority number, of those bridges the one with the lowest MAC address is designated as the root bridge.

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Contents Management Software Page Contents Contents Figures Figures Preface Where to Find Web-based Guides Management Software Updates Contacting Allied TelesisPreface Getting Started ChapterStarting a Management Session Main Save Configuration Click Apply Saving ChangesQuitting a Management Session Getting Started Basic Switch Parameters Basic Switch Parameters AT-S82 Management Software User’s Guide Disabling or Enabling Ping Blocking EnabledDisabled Enabling or Disabling 802.1X Forwarding Control Changing the Administrator’s Password Rebooting the Switch Resetting the Switch and Retaining the IP Address Reset Click Apply Switch is resetAT-S82 Management Software User’s Guide Downloading New Firmware Download Status MessagePort Configuration Configuring Port Parameters Port is enabled. This is the default Copper FiberTrunking Trunking Overview Trunking Guidelines Trunking AlgorithmAT-S82 Management Software User’s Guide Configuring the Trunking Algorithm DAxorSASetting up the Trunk Port Trunking ConfigurationTrunking Port Mirroring Port Mirroring Overview Configuring Port Mirroring Port Mirroring VLANs Vlan Features Increased Performance Improved Manageability SecurityAT-S82 Management Software User’s Guide Vlan Overview Vlan Identifier Vlan Port MembersIncoming Outgoing Tagged Untagged Frames Guidelines for Creating a Vlan Working with VLANs Vlan DetailDisplaying all VLANs Click Show All Static Vlan EntriesVlan Protected Ports Vlan Protected Ports Vlan ExampleAT-S82 Management Software User’s Guide VLANs Class of Service Class of Service Overview Mapping Ports to Egress QueuesIeee 802.1p Priority Level Port Priority Queue Port Egress Queue Maximum Number Packets Mapping Priorities to Queues Setting Up the Schedule Assigning Priority to Ports Class of Service Spanning Tree Protocol STP Spanning Tree Overview Bridge Priority RootPath Costs and Port Costs Increment Bridge PriorityPort Speed Port Cost Port Priority Increment Port PriorityHello Time and Bridge Protocol Data Units BPDUs Point-to-Point and Edge Ports Edge Port Spanning Tree and VLANs Spanning Tree Protocol STP Configuring Spanning Tree Rstp STP CompatibleDisabled Configuring STP Port Settings True YesFalse AutoSpanning Tree Protocol STP
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