Bridging, Switching, and Spanning Tree 161
bridge. The bridge with the lowest such cost of all bridges on that segment is called the
designated bridge. The interface on the designated bridge on that segment is placed in a
forwarding state.
Figure 4-16
Sample Bridged Network in Need of STP
All other interfaces are placed in a blocking state. Table 4-9 summarizes the reasons why
Spanning Tree places a port in forwarding or blocking state:
Table 4-9
Spanning Tree: Reasons for Forwarding or Blocking
Characterization of Port
Spanning Tree State
Explanation
All root bridge's ports
Forwarding
The root bridge is always the
designated bridge on all
connected segments.
Each nonroot bridge's root port
Forwarding
The root port is the port
receiving the lowest-cost
CBPDU from the root.
Each LAN's designated bridge
Forwarding
The bridge forwarding the
lowest-cost CBPDU onto the
segment is the designated
bridge.
All other ports
Blocking
The port is not used for
forwarding frames, nor are any
frames received on these
interfaces considered for
forwarding.
E0
E1
E1
E1
E0
E0
E0
E0
E1
B1
B2
E1
B5
B3
B4
ch04.fm Page 161 Monday, March 20, 2000 5:02 PM