System information

Transparent Bridging Technology Basics
Book Title
20-424
Figure 20-4 A Transparent Bridge Network After STA Is Run
The spanning-tree calculation occurs when the bridge is powered up and whenever a topology
change is detected. The calculation requires communication between the spanning-tree bridges,
which is accomplished through configuration messages (sometimes called bridge protocol data
units, or BPDUs). Configuration messages contain information identifying the bridge that is
presumed to be the root (root identifier) and the distance from the sending bridge to the root bridge
(root path cost). Configuration messages also contain the bridge and port identifier of the sending
bridge and the age of information contained in the configuration message.
Bridges exchange configuration messages at regular intervals (typically one to four seconds). If a
bridge fails (causing a topology change), neighboring bridges soon detect the lack of configuration
messages and initiate a spanning-tree recalculation.
All transparent bridge topology decisions are made locally. Configuration messages are exchanged
between neighboring bridges. There is no central authority on network topology or administration.
Frame Format
Transparent bridges exchange configuration messages and topology change messages.
Configuration messages are sent between bridges to establish a network topology. Topology change
messages are sent after a topology change has been detected to indicate that the STA should be rerun.
The IEEE 802.1d configuration message format is shown in Figure 20-5.
Figure 20-5 The Transparent Bridge Configuration Message Format
The fields of the transparent bridge configuration message are as follows:
• Protocol identifier—Contains the value 0.
• Version—Contains the value 0.
• Message type—Contains the value 0.
Bridge
2
Bridge
1
Z
YW
X
Bridge
5
Bridge
3
Bridge
4
V
Active port
Blocking port
V
Protocol
identifier
2
Version
1
Message
type
1
Message
age
2
Forward
delay
2
Maximum
age
2
Flags
1
Root ID
8
Root
path
cost
4
Bridge ID
8
Port ID
2
Hello
time
2
Field length,
in bytes