ZyXEL Communications NWA-1100 User Manual

Chapter 6 Wireless Settings Screen

ZyXEL NWA-1100 User’s Guide

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 - see the following 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.

STP assigns five port states (see next table) to eliminate packet looping. A bridge port is not 
allowed to go directly from blocking state to forwarding state so as to eliminate transient 
loops. 

Table 12   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

Table 13   STP Port States

Disabled

STP is disabled (default).

Blocking

Only configuration and management BPDUs are received and processed.

Listening

All BPDUs are received and processed.

Learning

All BPDUs are received and processed. Information frames are submitted to the 

learning process but not forwarded.

Forwarding

All BPDUs are received and processed. All information frames are received and 

forwarded.