Spanning Tree
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3.2 Rules for Creating the Tree
Structure
UM RedundConfig RSPL
Release 2.0 02/2013
3.2.2 Setting up the tree structure
The bridge with the smallest number for the bridge identifier is called the
root bridge. It is (or will become) the root of the tree structure.
The structure of the tree depends on the root path costs. Spanning Tree
selects the structure so that the path costs between each individual bridge
and the root bridge become as small as possible.
If there are multiple paths with the same root path costs, the bridge further
away from the root decides which port it blocks. For this purpose, it uses
the bridge identifiers of the bridge closer to the root. The bridge blocks the
port that leads to the bridge with the numerically higher ID (a numerically
higher ID is the logically worse one). If 2 bridges have the same priority,
the bridge with the numerically larger MAC address has the numerically
higher ID, which is logically the worse one.
If multiple paths with the same root path costs lead from one bridge to the
same bridge, the bridge further removed from the root uses the port
identifier of the other bridge as the last criterion
(see fig. 18)
. In the
process, the bridge blocks the port that leads to the port with the
numerically higher ID (a numerically higher ID is the logically worse one).
If 2 ports have the same priority, the port with the higher port number has
the numerically higher ID, which is logically the worse one.