Chapter 17 Spanning Tree Protocol

17.3.1 STP Terminology

The root bridge is the base of the spanning tree.

Path cost is the cost of transmitting a frame onto a LAN through that port. The recommended cost is assigned according to the speed of the link to which a port is attached. The slower the media, the higher the cost.

Table 27 STP Path Costs

 

LINK

RECOMMENDED

RECOMMENDED

ALLOWED

 

SPEED

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 bridge communicates with the root through the root port. The root port 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 Switch 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.

17.3.2 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 switches 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.

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