ZyXEL Communications 5 Series User Manual

Chapter 8 Bridge Screens

ZyWALL 5/35/70 Series User’s Guide

The root bridge is the base of the spanning tree. Path cost is the cost of transmitting a frame 
from the root bridge to 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 - see the next table.

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 bridge 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 bridges 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 33   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