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Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter14 Configuring STP
Understanding Spanning-Tree Features
Blocking State
A Layer 2 interface in the blocking state does not participate in frame forwarding. Af ter initialization, a
BPDU is sent to each interface in the switch. A switch initially functions as the root until it exchanges
BPDUs with other switches. This exchange establishes which switch in the network is the root or root
switch. If there is only one switch in the network, no exchange occurs, the forward-delay timer expires,
and the interfaces move to the listening state. An interface always enters the blocking state after switch
initialization.
An interface in the blocking state performs as follows:
•Discards frames received on the port
•Discards frames switched from another interface for forwardi ng
•Does not learn addresses
•Receives BPDUs
Listening State
The listening state is the first state a Layer 2 i nterface enter s after the blo cking s tate. T he interf ace e nters
this state when the spanning tree determines that the interface should participate in frame forwarding.
An interface in the listening state performs as follows:
•Discards frames received on the port
•Discards frames switched from another interface for forwardi ng
•Does not learn addresses
•Receives BPDUs
Learning State
A Layer 2 interface in the learning state prepares to participate in f rame for warding . The inte rface enters
the learning state from the listening state.
An interface in the learning state performs as follows:
•Discards frames received on the port
•Discards frames switched from another interface for forwardi ng
•Learns addresses
•Receives BPDUs
Forwarding State
A Layer 2 interface in the forwarding state forwards frames. The interface enters the forwa rding sta te
from the learning state.
An interface in the forwarding state performs as follows:
•Receives and forwards frames received on the port
•Forwards frames switched from another port
•Learns addresses
•Receives BPDUs