For the stable network topology, only the Root port and Designated port enter the
Forwarding status, while other ports are only in the Discarding status.
17.1.1.6 Spanning of Network Topology Tree
(Typical Application Solution)
We now describe how the STP and RSTP protocol spans the mixed network topology to a
tree-type structure. As is shown in Figure 17-4 below, the bridge IDs of the Switch A, B and
C are assumed to be increasing. Namely, the Switch A presents the highest priority. There is
the 1000M link between switch A and switch B, and the 100M link between the switch A and
switch C, while it is the 10M link between switch B and switch C. The Switch A acts as the
backbone switch of this network and implements the link redundancy for both Switch B and
Switch C. Obviously, it will generat the broadcast storm if all these links are active.
Figure 17-4
If all of three Switches enable the Spanning Tree protocol, they will select the root bridge as
the Switch A by switching the BPDU. 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. While, Switch C detects that it can reach A in the B to A way or
directly. However, the switch discovers that the Path cost in the B to A way is lower than that
directly (For the Path cost corresponding to various paths, refer to table ), so Switch C
selects the port connected with B as the Root port, while selects that connected with A as the
Alternate port. It will enter corresponding status of various ports to generate corresponding
Figure 17-5 after the port roles are selected.
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