ZyXEL Communications 4528F User Manual

Chapter 13 Spanning Tree Protocol

XGS-4526/4528F/4728F User’s Guide

Path cost is the cost of transmitting a frame onto a LAN through that port. The 
recommended cost 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 bridge communicates with the root through the root port. The 
root port 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. 

After a bridge determines the lowest cost-spanning tree with STP, it enables the 
root port and the ports that are the designated ports for connected LANs, and 
disables all other ports that participate in STP. Network packets are therefore only 
forwarded between enabled ports, eliminating any possible network loops.

STP-aware switches exchange Bridge Protocol Data Units (BPDUs) periodically. 
When the bridged LAN topology changes, a new spanning tree is constructed.

Once a stable network topology has been established, all bridges listen for Hello 
BPDUs (Bridge Protocol Data Units) transmitted from the root bridge. If a bridge 
does not get a Hello BPDU after a predefined interval (Max Age), the bridge 
assumes that the link to the root bridge is down. This bridge then initiates 
negotiations with other bridges to reconfigure the network to re-establish a valid 
network topology.

Table 26   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