Stp Port States - Cisco 6500 Series Software Configuration Manual

Understanding How STP Works
For example, assume that one port on Switch B is a fiber-optic link, and another port on Switch B (an
unshielded twisted-pair [UTP] link) is the root port. Network traffic might be more efficient over the
high-speed fiber-optic link. By changing the STP port priority on the fiber-optic port to a higher priority
(lower numerical value) than the root port, the fiber-optic port becomes the new root port.

STP Port States

These sections describe the STP port states:
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STP Port State Overview
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
a Layer 2 LAN port transitions directly from nonparticipation in the spanning tree topology to the
forwarding state, it can create temporary data loops. Ports 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 frames that have been forwarded using the old topology.
Each Layer 2 LAN port on a Catalyst 6500 series switch using STP exists in one of the following five
states:
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A Layer 2 LAN port moves through these five states as follows:
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Figure 15-2
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
15-6
STP Port State Overview, page 15-6
Blocking State, page 15-8
Listening State, page 15-9
Learning State, page 15-10
Forwarding State, page 15-11
Disabled State, page 15-12
Blocking—The Layer 2 LAN port does not participate in frame forwarding.
Listening—First transitional state after the blocking state when STP determines that the Layer 2
LAN port should participate in frame forwarding.
Learning—The Layer 2 LAN port prepares to participate in frame forwarding.
Forwarding—The Layer 2 LAN port forwards frames.
Disabled—The Layer 2 LAN port does not participate in STP and is not forwarding frames.
From initialization to blocking
From blocking to listening or to disabled
From listening to learning or to disabled
From learning to forwarding or to disabled
From forwarding to disabled
illustrates how a Layer 2 LAN port moves through the five states.
Chapter 15 Configuring STP and IEEE 802.1s MST
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