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DES-7200 Configuration Guide Chapter 10 MSTP Configuration
10-5
10.1.1.6 Generating a Network
Topology Tree (Typical
Application Solution)
We now describe how the STP and RSTP protocols span a tree-type structure
by the mixed network topology. As shown in Figure 4, the bridge IDs of Switches
A, B and C are assumed in the ascending order. Namely, Switch A presents the
highest priority. There is the 1000M link between switch A and switch B, and the
100M link between switch A and switch C, while it is the 10M link between
switch B and switch C. Switch A acts as the backbone switch of this network
and implements the link redundancy for both Switch B and Switch C. Obviously,
broadcast storm would occur if all these links are active.
Figure-4
If all of these three switches enable the Spanning Tree protocol, they will select
switch A as the root bridge by exchanging BPDU message. Once Switch B
detects that two ports are connected to Switch A, it will select the port with the
highest priority as the root port, while another one is selected as the alternate
port. Meanwhile, Switch C detects that it can reach Switch A through Switch B
or directly. However, Switch C discovers that the cost of the path from Switch B
to Switch A is lower than that directly (For the costs corresponding to various
paths, refer to table ***), so Switch C selects the port connected with Switch B
as the root port, while the one that connected with Switch A as the alternate port.
Various ports enter the corresponding status after their roles are determined. As
a result, the network topology is generated as shown in Figure 5.