Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 470)

Spanning Tree Introduction: STP, RSTP, and MSTP
Software Reference for x310 Series Switches
18.2 AlliedWare Plus TM Operating System - Version 5.4.4C C613-50046-01 REV A
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.