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3-2 C
HAPTER
3: A
DVANCED
B
RIDGING
DUA1860-0AAA01
Introduction
This chapter describes how to set up the bridge's advanced
features. If you have not previously set up the bridge, read
Chapter 2.
Spanning Tree Configuration
The
Spanning Tree Algorithm and Protocol
(STAP) is explained in
the IEEE Computer Society's
MAC Bridges P802.1D
document.
You should not change any spanning tree parameters unless you
have significant knowledge and experience with the Spanning
Tree Algorithm and Protocol (STAP).
The
spanning tree
eliminates the duplication of packets and
provides fault tolerance for resilient networks. During
construction of the spanning tree, bridges co-operate by
exchanging information. This information is transmitted in
packets called
Configuration Bridge Protocol Data Units
(C-BPDUs).
In order to establish a stable spanning tree topology, the STAP
bridges ’elect’ a
root bridge
. The root bridge takes charge of the
spanning tree topology and controls which bridges block packets
and which forward packets.
Once the topology is stable, all STAP bridges listen for special
’Hello’ C-BPDUs transmitted from the root bridge at regular
intervals (usually every two seconds). If a STAP bridge timer
expires before receiving a ‘Hello’ C-BPDU, it assumes that the root
bridge, or a link between itself and the root bridge, has gone
down. It initiates a reconfiguration of the spanning tree topology.
This section describes what spanning tree parameters can be
changed. The bridge defaults to the IEEE 802.1d Revision 9
recommended settings.