Intel ER8100STUS - Express 8100 Router, Reference Manual

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Reference.bok : Chapter10 109 Wed Apr 22 10:58:08 1998
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Spanning Tree Algorithm
WAN Bridging Services
If either of the two bridges fails, one of the network segments is isolated. To re-
duce the possibility of one of the bridges becoming isolated, it is common prac-
tice to add a third bridge—if any one of the bridges fails, all three network
segments are still accessible via the other two bridges.
Using basic bridges which do not implement the Spanning Tree Algorithm, this
configuration cannot be implemented—the bridges transfers received packets on
one link to the network segments connected to the other links. This means that
packets can continue to cycle within the network segments indefinitely—this is
known as a “bridging loop”. The receiver receives many copies of the same pack-
et.
A solution to this problem is described in the following section.
Spanning Tree Protocol
Since alternative paths are desirable for backup and other purposes, IEEE and
ISO have drawn up a proposal for a standard which offers a solution to “network
loops” problem. The solution is called “The Spanning Tree Protocol”, and is de-
scribed in IEEE document 802.1D “Local MAC Bridges”.
Spanning Tree Concept
Basic requirements
The following gives an overview of the basic requirements of the Spanning Tree
Algorithm for bridges. For full details, see IEEE 802.1D.
Bridge A
Bridge C
Bridge B
#1
#3
#2
Matchbox Router
Matchbox Router
Matchbox Router
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Matchbox Router
Matchbox Router
Matchbox Router