Cisco Systems DOC-7814982 Blocking State, Listening State, Learning State, Forwarding State

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Catalyst 2950 Desktop Switch Software Configuration Guide
78-14982-01
Chapter 11 Configuring STP Understanding Spanning-Tree Features
Blocking State
A Layer 2 in ter face in th e blo ck ing state does not partic ip ate in fr ame f o rw a rdin g. Af ter in iti aliz atio n , 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 the re is only one switch in the network, no e xch an ge oc curs, the for w ar d-d ela y timer e xpir es,
and the interfaces move to the listening state. An interface always enters the blocking state after switch
initialization.
An in terface in t he b lo cki ng state pe rfor ms as follows:
•Discards frames received on the port
•Discard s fr a mes sw it ched fr om anot h er in te rface fo r for war ding
•Does not learn addresses
•Rece ives BPDUs
Listening State
The li stening state i s the fir st state a Layer 2 inte rface enter s after the block ing sta te. The 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
•Discard s fr a mes sw it ched fr om anot h er in te rface fo r for war ding
•Does not learn addresses
•Rece ives BPDUs
Learning State
A Layer 2 interf ace in the learnin g state p repares to particip ate 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
•Discard s fr a mes sw it ched fr om anot h er in te rface fo r for war ding
•Learns addr esses
•Rece ives BPDUs
Forwarding State
A Layer 2 interface i n the forwa rding state forwar ds frames. The inte rface ente rs the fo rwardin g state
from the learning st ate.
An in terface in t he f orwa rdi ng state pe rfor ms as follows:
•Rece ives and for wards fr ames re ceived on the por t
•Forwards frames switched from another port
•Learns addr esses
•Rece ives BPDUs