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STP Terms and Concepts 175
4 Because the bridge now knows the root bridge, it can determine its
distance to the root and elect a root port.
It examines CBPDUs from all ports to see which port has received a
CBPDU with the smallest cost to the root. This port becomes the root
port.
5 Now that the bridge knows the contents of its own CBPDU, it can
compare this updated CBPDU with the ones that its other ports received.
■
If the bridge’s message is better than the ones received on any of its
ports, then the bridge assumes that it is the designated bridge for the
attached LANs.
■
If the bridge receives a better CBPDU on a port than the message it
would transmit, it no longer transmits CBPDUs on that LAN. When the
algorithm stabilizes, only the designated bridge transmits CBPDUs on
that LAN.
How Multiple Bridges
Interpret CBPDUs
The previous section looked at how a single bridge reviews CBPDUs and
makes decisions. The following examples illustrate how STP determines
the topology for an entire network.
Figure 5 and Figure 6 shows the same network topology — six bridges
that connect six LANs. The topology is designed with redundant links for
backup purposes, which create loops in the extended network. Figure 5
shows the network at the start of the STP topology calculation. Figure 6
shows the network after the STP topology has stabilized.