Chapter 10 IP Screen

NWA1100-N User’s Guide

93

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. 

10.5.2.3  How STP Works

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.

10.5.2.4  STP Port States

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 25   

STP Path Costs

LINK SPEED

RECOMMENDED 
VALUE

RECOMMENDED 
RANGE

ALLOWED 
RANGE

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 26   

STP Port States

PORT STATES

DESCRIPTIONS

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.