Chapter 11 IP Screen

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.

Table 35

STP Path Costs

 

 

 

 

 

LINK SPEED

RECOMMENDED

RECOMMENDED

ALLOWED

 

 

VALUE

RANGE

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

 

 

 

 

 

 

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.

11.5.2.3 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.

11.5.2.4 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 36 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.

 

 

108

 

NWA1100-N User’s Guide