Redundancy
12.3
Spanning Tree
164
UM Config GRS
Release
8.0
09/2019
12.3.3
Examples
Example of determining the root path
You can use the network plan
(see figure 37)
to follow the flow chart
(see figure 36)
for determining
the root path. The administrator has specified a priority in the bridge identification for each bridge.
The bridge with the smallest numerical value for the bridge identification takes on the role of the
root bridge, in this case, bridge 1. In the example every sub-path has the same path costs. The
protocol blocks the path between bridge 2 and bridge 3 as a connection from bridge 3 via bridge 2
to the root bridge would result in higher path costs.
The path from bridge 6 to the root bridge is interesting:
The path via bridge 5 and bridge 3 creates the same root path costs as the path via bridge 4 and
bridge 2.
STP selects the path using the bridge that has the lowest MAC address in the bridge
identification (bridge 4 in the illustration).
There are also 2 paths between bridge 6 and bridge 4.
The port identifier is decisive here (Port 1 < Port 3).
Figure 37: Example of determining the root path
Note:
When the current root bridge goes down, the MAC address in the bridge identifier alone
determines which bridge becomes the new root bridge, 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.
1
P-BID = 16 384
2
P-BID = 32 768
3
P-BID = 32 768
6
P-BID = 32 768
Port
3
7
P-BID = 32 768
P-BID = 32 768
Port
1
MAC 00:01:02:03:04:
06
5
P-BID = 32 768
4
Root path
Interrupted path
P-BID
Priority of the bridge identifikation (BID)
= BID without MAC Address
MAC 00:01:02:03:04:
05
Root Bridge
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