ZyXEL Communications ES-2108 User Manual

ES-2108 Series User’s Guide

Chapter 11 Spanning Tree Protocol

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This chapter introduces the Spanning Tree Protocol (STP).

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.

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.    

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. 

Table 20   STP Path Costs

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