Hirschmann MACH 1040 Series, User Manual

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Spanning Tree
50
4.4 Example of Root Path
Manipulation
Redundancy
Release 6.0 07/2010
4.4 Example of Root Path
Manipulation
The network plan (see fig. 18) can be used to create the flow diagram (see
fig. 17) for defining the root path. The Administrator – left the default value of
32,768 for each bridge except for bridge 1,– bridge 1 value was set to 16,384,
thus making it the root bridge. In the example, all the sub-paths have the
same path costs. The path between bridge 2 and bridge 3 is blocked by the
protocol because a connection from bridge 3 to the root bridge via bridge 2
has a higher path cost.
The path from bridge 6 to the root bridge is interesting:
X The path via bridge 5 and bridge 3 creates the same root path costs as
the path via bridge 4 and bridge 2.
X
STP selects the path using the bridge that has the lowest MAC address
in the bridge identification (bridge 4 in the illustration).
X
However, there are also 2 paths between bridge 6 and bridge 4. The port
identifier is decisive here.
Note: Because the Administrator does not change the default values for the
priorities of the bridges in the bridge identifier, apart from the value for the
root bridge, the MAC address in the bridge identifier alone determines which
bridge becomes the new root bridge if the root bridge becomes inoperable.