Specifications

Functions
104
4.4 Spanning Tree Algorithm
RS2-4R
Release 1.0 03/04
The port number and port priority number are stored in the Management
Information Base (see
“dot1dBridge (1.3.6.1.2.1.17)” on page 316):
- dot1dStpPortPathCost (1.3.6.1.2.1.17.2.15.10,1.Index)
- dot1dStpPortPriority (1.3.6.1.2.1.17.2.15.1.2.Index)
Fig. 33: Port identification
In order to compute their tree structures, the bridges need information about
other bridges that are present in the network. This information is obtained by
each bridge sending a BPDU (Bridge Protocol Data Unit) to all other bridges.
Along with other information, the BPDU contains the
D bridge identification,
D root path costs, and
D port identification
(see IEEE 802.1D).
D The bridge with the numerically smallest bridge identification is made the
root bridge. It forms the root of the tree structure.
D The structure of the tree depends upon the root path costs. The structure
that is chosen is the one that provides the lowest path costs between each
individual bridge and the root bridge.
D If there are multiple paths with the same root path costs, the priorities of
the bridge identifications for the bridges connected to this path determine
which bridge is blocked.
D If there are two paths leading away from a single bridge with the same root
path costs, the port identification is used as the last criterion for determi
-
ning which path is used (see Fig. 33). It decides which port is selected.
Priority Port number
MSB LSB