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Spanning Tree Introduction: STP, RSTP, MSTP
©2008 Allied Telesis Inc. All rights reserved.
18.2 AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E Software Version 5.2.1
Introduction
This chapter describes and provides configuration procedures for:
■
Spanning Tree Protocol (STP)
■ Rapid Spanning Tree Protocol (RSTP)
■ Multiple Spanning Tree Protocol (MSTP)
For detailed information about the commands used to configure spanning trees, see
Chapter 19, Spanning Tree Commands .
Overview of Spanning Trees
The concept of the spanning tree protocol was devised to address broadcast storming. The
spanning tree algorithm itself is defined by the IEEE standard 802.1D and its later revisions.
The IEEE Standard 802.1 uses the term “bridge” to define the spanning tree operation and uses
terms such as Bridge Protocol Data Units, Root Bridge etc., when defining spanning tree
protocol functions.
When a bridge receives a frame, it reads the source and destination address fields. The bridge
then enters the frame’s source address in its forwarding database. In doing this the bridge
associates the frame’s source address with the network attached to the port on which the
frame was received. The bridge also reads the destination address and if it can find this address
in its forwarding database, it forwards the frame to the appropriate port. If the bridge does not
recognize the destination address, it forwards the frame out from all its ports except for the
one on which the frame was received, and then waits for a reply. This process is known as
“flooding.”
A significant problem arises where bridges connect via multiple paths. A frame that arrives with
an unknown destination address is flooded over all available paths. The arrival of these frames
at another network via different paths and bridges produces major problems. The bridges can
become confused about the location of the send and receive devices and begin sending frames
in the wrong directions. This process feeds on itself and produces a condition known as a
broadcast storm, where the increase of circulating frames can eventually overload the network.
Spanning tree operation
Where a LAN’s topology results in more than one path existing between bridges, frames
transmitted onto the extended LAN circulate in increasing numbers around the loop,
decreasing performance and potentially overloading the network. However, multiple paths
through the extended LAN are often required in order to provide redundancy and backup in
the event of a bridge or link failure.
The spanning tree is created through the exchange of Bridge Protocol Data Units (BPDUs)
between the bridges in the LAN. The spanning tree algorithm operates by:
■
Automatically computing a loop-free portion of the topology, called a spanning tree . The
topology is dynamically pruned to the spanning tree by declaring certain ports on a switch
to be redundant, and placing them into a ‘Blocking’ state.
■ Automatically recovering from a switch failure that would partition the extended LAN by
reconfiguring the spanning tree to use redundant paths, if available.