Cisco Cisco Aironet 340 Ethernet Bridges 用户指南
Using the Spanning-Tree Protocol 8 - 5
Each bridge starts by assuming it is the root and its root cost is 0. When
a bridge receives a configuration message, it records the values only if
the message received is better than the message it would transmit out the
port.
a bridge receives a configuration message, it records the values only if
the message received is better than the message it would transmit out the
port.
For example, message C1 is better than C2:
n If the root ID in C1 has a lower numeric value than the value from
C2.
n If the root ID’s are equal and C1’s root cost is lower.
n If the root ID’s and costs are equal and C1’s transmitting Bridge ID
has a lower numeric value.
n If the root ID, cost, and Bridge ID are equal and C1 was transmitted
on a port with a lower port ID. This should only occur if two ports
from the same bridge are connected to the same LAN.
from the same bridge are connected to the same LAN.
If a port receives a better message than the one it would transmit, the
bridge stops transmitting configuration messages on that port. Only one
port on each LAN will be transmitting the messages. The bridge that
contains this port is called the designated bridge for that LAN and the
port is called the designated port.
bridge stops transmitting configuration messages on that port. Only one
port on each LAN will be transmitting the messages. The bridge that
contains this port is called the designated bridge for that LAN and the
port is called the designated port.
Determining the Root Bridge and Root Cost
Each bridge determines the root bridge’s ID by comparing its own ID
with those from the best messages received on all of its ports. The root
ID is then used in all transmitted configuration messages.
with those from the best messages received on all of its ports. The root
ID is then used in all transmitted configuration messages.
If a bridge is the root, its root cost is 0. If a bridge is not a root, its cost
is the minimum of the costs received in the messages from all its ports
as well as the cost from the port on which the minimum cost message
was received. This cost is then used in all transmitted configuration
messages.
is the minimum of the costs received in the messages from all its ports
as well as the cost from the port on which the minimum cost message
was received. This cost is then used in all transmitted configuration
messages.
The port on which the minimum cost message was received is called the
root port.