Cisco Cisco Aironet 340 Ethernet Bridges 사용자 가이드

The STP protocol works by having the bridges transmit special configu-
ration messages to each other. The messages contain enough information 
to allow the bridges to:

n Elect a single bridge. A single bridge is selected, from all the 

bridges on all the LAN, to be the root bridge. Each bridge then 
calculates the distance of the shortest path to the root bridge.

n Elect a designated bridge (for each LAN). A bridge from all the 

bridges residing on the LAN will be selected. This bridge will be 
closest to the root bridge.

n Select its own port to be root port.This bridge has the best path to 

the root bridge.

n Select ports are to be included in the spanning tree. Ports are 

included if they are a root port or the bridge itself has been selected 
as the designated bridge for the port’s LAN. 

Any ports not included in the spanning tree will be blocked and any data 
received from its LAN will be discarded. The bridge will not forward 
any traffic to this port.

Configuration messages contain four main fields.

n The Bridge ID of the root bridge. This is called the root ID. A 

bridge’s ID consists of a 16 bit priority value appended with the 
infrastructure address of the bridge. The infrastructure address of 
the bridge is usually the address of one of the bridge’s ports.The pri-
ority value is assigned by the operator with a default value of 8000 
hex. 

n The Bridge ID of the transmitting bridge.

n The cost of the path from the transmitting bridge to the root bridge.

n The port ID of the port on which the message was transmitted.The 

ID is made up of an 8 bit priority value appended with an 8 bit port 
number assigned to the port by the bridge. The priority value is 
assigned by the operator with a default value of 80 hex.