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Understanding How STP Works

Understanding How STP Works
These sections describe how STP works:

STP Overview

STP is a Layer 2 link management protocol that provides path redundancy while preventing undesirable
loops in the network. For a Layer 2 Ethernet network to function properly, only one active path can exist
between any two stations. STP operation is transparent to end stations, which cannot detect whether they
are connected to a single LAN segment or a switched LAN of multiple segments.
Catalyst 6500 series switches use STP (the IEEE 802.1D bridge protocol) on all VLANs. By default, a
single instance of STP runs on each configured VLAN (provided you do not manually disable STP). You
can enable and disable STP on a per-VLAN basis.
When you create fault-tolerant internetworks, you must have a loop-free path between all nodes in a
network. The STP algorithm calculates the best loop-free path throughout a switched Layer 2 network.
Layer 2 LAN ports send and receive STP frames at regular intervals. Network devices do not forward
these frames, but use the frames to construct a loop-free path.
Multiple active paths between end stations cause loops in the network. If a loop exists in the network,
end stations might receive duplicate messages and network devices might learn end station MAC
addresses on multiple Layer 2 LAN ports. These conditions result in an unstable network.
STP defines a tree with a root bridge and a loop-free path from the root to all network devices in the
Layer 2 network. STP forces redundant data paths into a standby (blocked) state. If a network segment
in the spanning tree fails and a redundant path exists, the STP algorithm recalculates the spanning tree
topology and activates the standby path.
When two Layer 2 LAN ports on a network device are part of a loop, the STP port priority and port path
cost setting determine which port is put in the forwarding state and which port is put in the blocking
state. The STP port priority value represents the location of a port in the network topology and how well
located it is to pass traffic. The STP port path cost value represents media speed.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
15-2
STP Overview, page 15-2
Understanding the Bridge ID, page 15-3
Understanding Bridge Protocol Data Units, page 15-4
Election of the Root Bridge, page 15-4
STP Protocol Timers, page 15-5
Creating the Spanning Tree Topology, page 15-5
STP Port States, page 15-6
STP and IEEE 802.1Q Trunks, page 15-12
Chapter 15
Configuring STP and IEEE 802.1s MST
78-14099-04

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