Chapter 7

Spanning Tree Protocol


Spanning Tree Protocol

Using the Spanning Tree Protocol (STP) functionality of the FS3-1600 makes your network more intelligent for achieving network fault tolerance. The following sections explain more about STP and the STP features supported by the switch.

STP is a part of the 802.1D bridge specification defined by the IEEE Computer Society. To explain STP in terms used by the 802.1D specification, the FS3-1600 switch will be referred to as a multi-port bridge. All STP devices will communicate with each other via Bridge Protocol Data Unit (BPDU) frames.

STP Overview

STP is a bridge-based mechanism for providing fault tolerance on networks. STP is a protocol used by bridges (or switches) to remove redundant links from networks. All spanning tree devices (including FS3-1600) will send BPDU frames out each of their ports and listen for BPDU frames from other devices. The protocol allows spanning tree devices to communicate path cost and identification information so that each device can block the highest cost path that is redundant. Therefore, STP allows you to implement parallel paths for network traffic, and ensure the following functionality:

 


State Transition of STP

Each switch is identified by a switch ID, and each switch port (interface) on a switch is identified by a Port ID. The state transition of STP protocol is described in Figure 7- 1.

Ports can be either disabled or enabled. Ports that are enabled can be in one of the following states:

Figure 7- 1: State Transition of STP Protocol


Spanning Tree Topology

The path cost factor is used to calculate the distance from each port of a switch to the Root Switch. The following describes the roles of switches and ports, see Figure 7- 2.

Figure 7- 2: The Roles in Spanning Tree Topology

 


An Example of Building Redundant Links

It is most useful to build redundant links in preparation for emergencies. For example, in Figure 7- 3, the main link (“M” link) to an e-mail server was kicked out by a careless system administrator, the redundant link (“R” Link) could be brought up to operate while the main link was being fixed. STP solves this emergency automatically. The network checks to make sure that all of the links are still valid, and if one goes down, the next link that serves the disabled segment will be brought up to replace it.

Figure 7- 3: An Example of Building Redundant Link

A STP is designed for a set of bridges and switches, which has a loop in the network. The links that are taken down are always the lowest priority links (as configured by the network administrator). You must be careful when placing spanning tree on a multi-port device (such as an Ethernet switch), which makes use of virtual LANs. Most vendors do not allow spanning tree to coexist with virtual networking.

 

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