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Spanning Tree Protocol 5-15
An Example
Figure 5-13
illustrates part of a network. All bridges
have a path cost value assigned to each port, identi-
fied by PC=xxx (where xxx is the value).
Bridge A is selected by STP as the Root Bridge,
because it has the lowest Bridge Identifier. The Des-
ignated Bridge Port for LAN A is port 1 on Bridge A.
Each of the other four bridges have a Root Port (the
port closest to the Root Bridge). Bridge X and Bridge
B can offer the same path cost to LAN B. In this
case Bridge B's port is chosen as the Designated
Bridge Port, because it has the lowest Bridge Identi-
fier. Bridge C's port is chosen as the Designated
Bridge Port for LAN C because it offers the lowest
Root Path Cost (the route through Bridge C and B
costs 200, the route through Bridge Y and B would
cost 300). You can set the path cost of a bridge
port to influence the configuration of a network
with a duplicate path.
Once the network topology is stable, all the bridges
listen for special Hello BPDUs transmitted from the
Root Bridge at regular intervals. If the STP Max Age
time expires (refer to “
Configuring the STP Parame-
ters of VLANs” on page 5-18) before receiving a
Hello BPDU, it assumes that the Root Bridge, or a
link between itself and the Root Bridge, has gone
down. It then initiates a reconfiguration of the net-
work topology.
You can adjust timers to determine how quickly a
network reconfigures and therefore how rapidly the
network recovers from a path failure (refer to “
Con-
figuring the STP Parameters of VLANs” on page
5-18).
Figure 5-13
Port costs in a network