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How Spanning Tree Protocols Work
The Spanning Tree Protocol (STP) uses a distributed algorithm that selects one bridge of a redundantly
connected network as the root of a spanning tree connected active topology. STP assigns roles to each
port depending on what the port's function is in the active topology. Port roles are as follows:
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Switches that have ports with these assigned roles are called root or designated switches. See the next
section,
In Ethernet networks, only one active path may exist between any two stations. Multiple active paths
between stations can cause loops in the network. When loops occur, some switches recognize stations
on both sides of the switch. This situation causes the forwarding algorithm to malfunction allowing
duplicate frames to be forwarded.
Spanning tree algorithms provide path redundancy by defining a tree that spans all of the switches in an
extended network and then forces certain redundant data paths into a standby (blocked) state. At regular
intervals the switches in the network send and receive spanning tree packets which they use to identify
the active path. If one network segment becomes unreachable, or if spanning tree costs change, the
spanning tree algorithm reconfigures the spanning tree topology and reestablishes the link by activating
a standby path.
Spanning tree operation is transparent to end stations, which do not detect whether they are connected
to a single LAN segment or a switched LAN of multiple segments.
How a Topology Is Created
All switches in an extended LAN participating in a spanning tree gather information about other
switches in the network through an exchange of data messages known as bridge protocol data units
(BPDUs). This exchange of messages results in the following actions:
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The topology of an active switched network is determined by the following:
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In a switched network, the root switch is the logical center of the spanning tree topology. A spanning
tree protocol uses BPDUs to elect the root switch and root port for the switched network, as well as the
root port and designated port for each switched segment.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
7-2
Root—A unique forwarding port elected for the spanning tree topology
Designated—A forwarding port elected for every switched LAN segment
Alternate—A blocked port providing an alternate path to the root port in the spanning tree
Backup—A blocked port in a loopback configuration
How a Topology Is
Created.
A unique root switch is elected for the spanning tree network topology.
A designated switch is elected for every switched LAN segment.
Any loops in the switched network are eliminated by placing redundant switch ports in a backup
state; all paths that are not needed to reach the root switch from anywhere in the switched network
are placed in STP-blocked mode.
The unique switch identifier (MAC address of the switch) associated with each switch
The path cost to the root associated with each switch port
The port identifier (MAC address of the port) associated with each switch port
Chapter 7
Configuring Spanning Tree
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