Spanning-tree interface states, Figure 7-1 – Cisco 15327 User Manual
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Ethernet Card Software Feature and Configuration Guide, R7.2
Chapter 7 Configuring STP and RSTP
Spanning-Tree Interface States
Figure 7-1
Spanning-Tree Topology
When the spanning-tree topology is calculated based on default parameters, the path between source and
destination end stations in a switched network might not be ideal. For instance, connecting higher-speed
links to an interface that has a higher number than the root port can cause a root-port change. The goal
is to make the fastest link the root port.
Spanning-Tree Interface States
Propagation delays can occur when protocol information passes through a switched LAN. As a result,
topology changes can take place at different times and at different places in a switched network. When
an interface transitions directly from nonparticipation in the spanning-tree topology to the forwarding
state, it can create temporary data loops. Interfaces must wait for new topology information to propagate
through the switched LAN before starting to forward frames. They must allow the frame lifetime to
expire for forwarded frames that have used the old topology.
Each Layer 2 interface on a switch using spanning tree exists in one of these states:
•
Blocking—The interface does not participate in frame forwarding.
•
Listening—The first transitional state after the blocking state when the spanning tree determines
that the interface should participate in frame forwarding.
•
Learning—The interface prepares to participate in frame forwarding.
•
Forwarding—The interface forwards frames.
•
Disabled—The interface is not participating in spanning tree because of a shutdown port, no link on
the port, or no spanning-tree instance running on the port.
An interface moves through these states:
1.
From initialization to blocking
2.
From blocking to listening or to disabled
3.
From listening to learning or to disabled
4.
From learning to forwarding or to disabled
5.
From forwarding to disabled
83803
DP
DP
RP
DP
DP
RP
DP
RP = root port
DP = designated port
DP
RP
DP
D
A
C
B
ONS 15454
with ML100T-12
ONS 15454
with ML100T-12
ONS 15454
with ML100T-12
ONS 15454
with ML100T-12