10-13
Bridging—Transmitting Non-IP Traffic or Merging Two Networks
Configuring Spanning Tree
A device then marks the following ports for activation (forwarding frames):
■ the root port
■ designated ports—which connect to devices that consider the local device
as their designated switch (and ports that connect to end users)
All other ports become inactive.
The root device periodically sends BPDUs. If the router is root, these BPDUs
will consume some bandwidth. Other devices only send topology change
notification BPDUs (TCN BPDUs).
When a device receives a TCN BPDU, it re-evaluates which ports are marked
for activation. If necessary, it transmits its own TCN BPDU, informing other
devices on the change. The port (or ports) through which the device transmits
a BPDU is not necessarily the one that received the BPDU that prompted the
change.
Devices determine which ports process BPDUs, learn information about the
network topology, forward BPDUs, and forward network traffic according to
the ports’ STP state.
STP States
STP includes the following port states:
■ disabled
■ blocking
■ listening
■ learning
■ forwarding
In a stable network, all ports are in either the forwarding or blocking state.
Only ports in the forwarding state forward frames. Ports in the blocking state
are not considered part of the network topology.
Note When using STP, it is important to understand the difference between disabled
and blocking ports. Neither type forwards frames or learns addresses. Neither
processes or transmits BPDUs. However, blocking ports receive BPDUs, while
disabled ports do not. If you disable a port, it will not participate in STP at all.
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