Specifications

Functions
RS2-4R
Release 1.0 03/04
4.4 Spanning Tree Algorithm
103
U Root path costs
Each path connecting two bridges has transmission costs assigned to it.
The management administrator sets this value and specifies it for each
path when configuring a bridge
(see Table 7 on page 109).
Because the management administrator essentially has a free hand in
specifying this value, he has a tool for ensuring that in case of redundant
paths one path will be favored over the others.
The root path costs are calculated by adding up the individual path costs
for the paths that a data packet must traverse between the port of a
bridge and the root bridge.
The root path costs and individual path costs are stored in the Manage-
ment Information Base (see “dot1dBridge (1.3.6.1.2.1.17)” on page 316):
- dot1dStpRootCost (1.3.6.1.2.1.17.2.6.0)
- dot1dStpPortPathCost (1.3.6.1.2.1.17.2.15.1.5.Index)
Fig. 32: Path costs
U Port identification
The port identification consists of two parts of 8 bits each. One part, the
lower-value byte, reflects a fixed relationship to the physical port number.
This part ensures that no port in a bridge receives the same designation
as another port in the same bridge. The second part contains the priority
number which is set by the management administrator. It is also true
here that the port with the lowest numerical value for its port identifier is
the one with the highest priority.
Ethernet (10 Mbit/s)
Bridge 2
Bridge 3
Bridge 1
X.21 (64 kbit/s)
PC Path costs
PC = 15 625
PC = 100
PC = 100