ES-2108/ES-2108-G User’s Guide

CHAPTER 11

Spanning Tree Protocol

This chapter introduces the Spanning Tree Protocol (STP).

11.1 Overview

STP detects and breaks network loops and provides backup links between switches, bridges or routers. It allows a switch to interact with other STP-compliant switches in your network to ensure that only one route exists between any two stations on the network.

11.1.1 STP Terminology

The root bridge is the base of the spanning tree; it is the bridge with the lowest identifier value (MAC address).

Path cost is the cost of transmitting a frame onto a LAN through that port. It is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost.

Table 20 STP Path Costs

 

LINK SPEED

RECOMMENDED VALUE

RECOMMENDED

ALLOWED RANGE

 

 

 

RANGE

 

Path Cost

4Mbps

250

100 to 1000

1 to 65535

 

 

 

 

 

Path Cost

10Mbps

100

50 to 600

1 to 65535

 

 

 

 

 

Path Cost

16Mbps

62

40 to 400

1 to 65535

 

 

 

 

 

Path Cost

100Mbps

19

10 to 60

1 to 65535

 

 

 

 

 

Path Cost

1Gbps

4

3 to 10

1 to 65535

 

 

 

 

 

Path Cost

10Gbps

2

1 to 5

1 to 65535

 

 

 

 

 

On each bridge, the root port is the port through which this bridge communicates with the root. It is the port on this switch with the lowest path cost to the root (the root path cost). If there is no root port, then this switch has been accepted as the root bridge of the spanning tree network.

For each LAN segment, a designated bridge is selected. This bridge has the lowest cost to the root among the bridges connected to the LAN.

Chapter 11 Spanning Tree Protocol

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