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8 - 4 Aironet 340 Series Bridge
How STP Protocol Works
The STP protocol works by having the bridges transmit special configuration
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.
Receiving Configuration Messages
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 priority
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.