Understanding How Spanning Tree Protocols Work; Understanding How A Topology Is Created - Cisco WS-X6066-SLB-APC - Content Switching Module Software Manual

Understanding 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. For more
information, see the
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 that they use to identify the
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 the 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.
These sections describe the STP:
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Understanding 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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Catalyst 6000 Family Software Configuration Guide—Releases 6.3 and 6.4
8-2
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
"Understanding How a Switch Becomes the Root Switch" section on page
Understanding How a Topology is Created, page 8-2
Understanding How a Switch Becomes the Root Switch, page 8-3
Understanding How Bridge Protocol Data Units Work, page 8-3
Calculating and Assigning Port Costs, page 8-4
Spanning Tree Port States, page 8-5
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 Media Access Control ([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 8 Configuring Spanning Tree
8-3.
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