ZyXEL Communications NWA-3163 & NWA-3166 User Manual

Chapter 8 Wireless Screen

NWA-3160 Series User’s Guide

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

The NWA uses IEEE 802.1w RSTP (Rapid Spanning Tree Protocol) that allow faster 
convergence of the spanning tree (while also being backwards compatible with 
STP-only aware bridges). Using RSTP topology change information does not have 
to propagate to the root bridge and unwanted learned addresses are flushed from 
the filtering database. In RSTP, the port states are Discarding, Learning, and 
Forwarding.

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 - 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. 

Table 27   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