3Com 4007, Implementation Manual, Page: 170 / 684

170 C
HAPTER
9: B
RIDGE
-W
IDE AND
B
RIDGE
P
ORT
P
ARAMETERS
STP Terms and
Concepts
This section provides a conceptual review of STP terms, parameters, and
processes. To learn how you can manipulate STP parameters, see “STP
Bridge and Port Parameters” in this chapter.
Configuration
Messages
In order to determine a loopless LAN topology, bridges that support STP
must communicate and share information with each other. STP Bridges
periodically transmit special frames called
Configuration Bridge Protocol
Data Units (CBPDUs), which contain several pieces of information that
help to determine the LAN topology. CBPDUs are stored in bridge
memory but are refreshed periodically with the latest information.
STP uses an algorithm that compares the information from different
CBPDUs to determine all possible paths and dynamically map out a
loopless network topology. STP keeps one bridge port active and puts
redundant bridge ports in the
blocking
state. A port in the blocking state
neither forwards nor receives data frames.
After STP logically eliminates the redundant paths, the network
configuration stabilizes. Thereafter, if one or more of the bridges or
communication paths in the stable topology fail, STP recognizes the
changed configuration and, within a few seconds, consults the stored
CBPDU information and activates an appropriate number of redundant
links to ensure that network connectivity is maintained.
CBPDUs do not propagate through the bridge as regular data frames do.
Instead, each bridge acts as an end station — receiving, interpreting, and
acting on the information in the CBPDUs.
Bridge Hierarchy
The CBPDUs help bridges establish a hierarchy (or a calling order ) among
themselves for the purposes of creating a loopless network.
Based on the information in the CBPDUs, the bridges elect a root bridge ,
which is at the top level of the hierarchy. The bridges then choose the
best path on which to transmit information to the root bridge.