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February 2010
3.9
Spanning Tree Algorithm / Redundancy
The bridge “Spanning Tree Protocol” function was introduced to handle network loops and provide
redundant paths in networks. To enable the STP requires that WDS mode also be enabled on the
“Repeaters” configuration page. When enabling WDS mode on a client/station you should ensure
that WDS mode is also enabled at the corresponding Access Point – refer section 3.10 for more
information.
For example, consider this network with a redundant wireless link. If the bridge Spanning Tree
Protocol is enabled, one of the two wireless links will be disabled - that is, all wireless data will be
transferred by one link only. If the active link fails, the other link will automatically start
transferring the wireless data.
The Spanning Tree Protocol implemented is IEEE 802.1d compatible. The algorithm forms a loop-
free network by blocking traffic between redundant links in the network. These blocked links are
placed in a standby condition, and may be automatically enabled to repair the network if another
link is lost. The Spanning Tree Algorithm maintains a single path between all nodes in a network,
by forming a tree-like structure. The Bridge Priority determines where the node sits in the tree. A
Bridge configured with the lowest priority (0) will become the root node in the network, and will
direct traffic between each of its branches. The root node is typically the unit that handles the
majority of traffic in the network. The WI-MOD-E is configured with a Bridge Priority of 32768 by
default. The intention is to reduce traffic that the WI-MOD-E must handle, by placing it at the
branch level in the network tree. As a branch, the WI-MOD-E needs only pass traffic to devices that
are its “leaves”.
When the bridge is turned on it needs to determine who the root bridge is and compute the port roles
(root, designated, or blocked). To ensure that each bridge has enough information, the bridges use
special data frames called
Bridge Protocol Data Units
(BPDUs) to exchange information about
bridge IDs and root path costs, etc.
There are some standard spanning tree protocol timers that can be adjusted that may help with
managing spanning tree Protocol. See section
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‘
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/ WDS Configuration’ below for details
There is some overhead in maintaining a network utilizing the Spanning Tree Algorithm. Users
wishing to increase their throughput, at the expense of redundancy should disable Spanning Tree.
The Spanning Tree Protocol can be configured on the
Repeaters
configuration page – note that 4-
address mode
must
be enabled if the bridge Spanning Tree Protocol is to be used (refer section “3.7
Normal Operation” for details).
