Spanning Tree Protocol 5-15
An Example
Figure 5-11
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 Designated 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 Identifier. 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 has a cost of
200, the route through Bridge Y and B has a cost of
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 of a bridge expires (refer to “Configuring the
STP Parameters of VLANs” on page 5-18) before
receiving a Hello BPDU, the bridge assumes that
the Root Bridge, or a link between itself and the
Root Bridge, has gone down. The bridge then ini-
tiates a reconfiguration of the network 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 5-11
Port costs in a network
